android_ab: Fix ANDROID_AB_BACKUP_OFFSET

Currently, setting CONFIG_AB_BACKUP_OFFSET in a target's defconfig will
not actually enable the #if protected code in android_ab.c. This is
because "CONFIG_" should have been prepended to the config macro, or the
macros defined in kconfig.h could have been used.

The code included by ANDROID_AB_BACKUP_OFFSET has been refactored to no
longer be conditionally compiled by preprocessor conditionals and
instead use C conditionals. This better aligns with the Linux kernel
style guide.

Fixes: 3430f24b ("android_ab: Try backup booloader_message")
Signed-off-by: Colin McAllister <colin.mcallister@garmin.com>
Cc: Joshua Watt <JPEWhacker@gmail.com>
Cc: Simon Glass <sjg@chromium.org>
Signed-off-by: Colin McAllister <colinmca242@gmail.com>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Reviewed-by: Igor Opaniuk <igor.opaniuk@gmail.com>
Reviewed-by: Sam Protsenko <semen.protsenko@linaro.org>
Link: https://lore.kernel.org/r/20240312125729.82695-3-colinmca242@gmail.com


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