rpi-eeprom-update: Add hook/flashrom help and fix section formatting

Document the EEPROM_CONFIG_HOOK option and move the existing
USE_FLASHROM env var to the new section.

The other more obscure path options can be documented in other commits

rpi-eeprom-update

@@ -291,34 +291,30 @@ checkDependencies() {
 usage() {
 cat <<EOF
 rpi-eeprom-update [options]... [FILE]
-   Checks whether the Raspberry Pi bootloader and the VL805 USB controller
-   EEPROMs are up-to-date and optionally updates the EEPROMs at the next reboot.
 
-   The default update mechanism writes recovery.bin and the EEPROM update 
+Checks whether the Raspberry Pi bootloader and the VL805 USB controller
-   image(s) (pieeprom.upd and vl805.bin) to the boot partition on the sd-card. 
+EEPROMs are up-to-date and optionally updates the EEPROMs at the next reboot.
-   The SHA256 hash of the corresponding images are written to pieeprom.sig
-   and/or vl805.sig. This guards against file system corruption which could
-   cause the EEPROM to be flashed with an invalid image. This is not a
-   security check.
 
-   At the next reboot the ROM runs recovery.bin which updates EEPROM(s). 
+The default update mechanism writes recovery.bin and the EEPROM update
-   If the update was successful recovery.bin renames itself to recovery.000
+image(s) (pieeprom.upd and vl805.bin) to the boot partition on the sd-card.
-   to prevent it from running a second time then resets the system.
+The SHA256 hash of the corresponding images are written to pieeprom.sig
-   The system should then boot normally.
+and/or vl805.sig. This guards against file system corruption which could
+cause the EEPROM to be flashed with an invalid image. This is not a
+security check.
 
-   If /boot does not correspond to the boot partition on the sd-card and this
+At the next reboot the ROM runs recovery.bin which updates EEPROM(s).
-   is not a NOOBS system then the mount point for BOOTFS should be defined
+If the update was successful recovery.bin renames itself to recovery.000
-   in /etc/default/rpi-eeprom-update by defining the BOOTFS variable.
+to prevent it from running a second time then resets the system.
+The system should then boot normally.
 
-   For reference, the flashrom update mechanism may be enabled by defining
+If /boot does not correspond to the boot partition on the sd-card and this
-   USE_FLASHROM=1 in /etc/default/rpi-eeprom-update. This not recommended
+is not a NOOBS system then the mount point for BOOTFS should be defined
-   because the SPI pins are muxed with audio and other device drivers may
+in /etc/default/rpi-eeprom-update by defining the BOOTFS variable.
-   be using SPI (e.g. HATs). This is also not safe in the event of a power
-   failure during the update of the EEPROM.
 
-   A backup of the current EEPROM config file is written to ${FIRMWARE_BACKUP_DIR}
+A backup of the current EEPROM config file is written to ${FIRMWARE_BACKUP_DIR}
-   before applying the update.
+before applying the update.
 
+Options:
    -a Automatically install bootloader and USB (VLI) EEPROM updates.
    -A Specify which type of EEPROM to automatically update (vl805 or bootloader)
    -d Use the default bootloader config instead of migrating the current settings
@@ -334,20 +330,42 @@ rpi-eeprom-update [options]... [FILE]
    -r Removes temporary EEPROM update files from the boot partition.
    -u Install the specified VL805 (USB EEPROM) image file.
 
-   To extract the configuration file from an EEPROM image: 
+Environment:
+Environment variables should be defined in /etc/default/rpi-eeprom-update
+
+EEPROM_CONFIG_HOOK
+
+Specifies the path of an optional script which post-processes the
+configuration file before it is applied to the new image. The modified
+output must contain at least 3 lines and should contain WAKE_ON_GPIO
+and POWER_OFF_ON_HALT settings.
+
+USE_FLASHROM
+
+The flashrom update mechanism may be enabled by setting USE_FLASHROM=1. This
+also selects the vl805 tool instead of using recovery.bin to perform the
+update. This may be desirable if an immediate update is required or if an
+sd-card is not present.
+However, this not recommended because the SPI pins are muxed with audio and other
+device drivers may be using SPI (e.g. HATs). This is also not safe in the
+event of a power failure during the update of the EEPROM.
+Changing the VL805 firmware whilst USB devices are attached may also cause
+those devices to stop working until after the system is reboot.
+
+Examples:
+To extract the configuration file from an EEPROM image:
    rpi-eeprom-config pieeprom.bin --out bootconf.txt
 
-   To update the configuration file in an EEPROM image: 
+To update the configuration file in an EEPROM image:
    rpi-eeprom-config pieeprom.bin --config bootconf.txt --out pieeprom-new.bin
 
-   To flash the new image: 
+To flash the new image:
    sudo rpi-eeprom-update -d -f ./pieeprom-new.bin
 
-   The syntax is the same as config.txt but section filters etc are not supported. See
+The syntax is the same as config.txt See online documentation for the list of parameters.
-   online documentation for the list of parameters.
 
-   The official documentation for the Raspberry Pi bootloader EEPROM is available at
+The official documentation for the Raspberry Pi bootloader EEPROM is available at
-   https://www.raspberrypi.org/documentation/hardware/raspberrypi/booteeprom.md
+https://www.raspberrypi.org/documentation/hardware/raspberrypi/booteeprom.md
 
 EOF
   exit ${EXIT_SUCCESS}