rpi-eeprom-config: Make padding more robust.

Make it explicit that a modifiable file is stored withing a
single 4K sector (for erase) and that this includes the 20
byte header.

When modifying a file pad up to the next section instead of
just to an alignment size. This enables future changes to be
more flexible in terms of alignment and padding. Although,
files/sections with different padding requirements will likely
get a different magic.

test/test-rpi-eeprom-config

@@ -135,7 +135,7 @@ check_conf_size_large()
 {
    echo "check maximum config file size"
    image="${script_dir}/$1"
-   conf="bootconf-2024.txt"
+   conf="bootconf-4076.txt"
 
    expected_md5="$(md5sum "${conf}" | awk '{print $1}')"
 
@@ -154,7 +154,7 @@ check_conf_size_too_large()
 {
    echo "check config file which exceeds the maximum size"
    image="${script_dir}/$1"
-   conf="bootconf-2025.txt"
+   conf="bootconf-4077.txt"
 
    expected_md5="$(md5sum "${conf}" | awk '{print $1}')"