android_ab: Fixes: Fix backup offset calculation

The backup offset is in bytes, but was incorrectly be interpreted as
blocks, leading to it being written to the wrong location. Fix the
calculation, clarify that ANDROID_AB_BACKUP_OFFSET is in bytes and must
be a multiple of the block size, and add a runtime check to validate the
offset.

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Fixes: 3430f24b ("android_ab: Try backup booloader_message")
Link: https://lore.kernel.org/r/20240828143924.3987331-1-JPEWhacker@gmail.com


Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>