Make it clearer that rpi-eeprom-digest is used create optionally
signed hashes for binary files.
For secure-boot the update-pieeprom.sh wrapper must be used
because only the boot configuration is signed by the customer
key rather than the entire SPI flash image.
Sync to the version from usbboot in order to add support for
the verify option. This provides a command line interface for
verifying a .sig file (with RSA) against a public key in .PEM
format.
