common: android_ab: fix slot suffix for abc block

common: android_ab: fix slot suffix for abc block

To align with the official Android BCB (Bootloader Control Block)
specifications, it's important to note that the slot_suffix should start
with an underscore symbol.

For a comprehensive understanding of the expected slot_suffix format in
userspace, please refer to the provided reference [1].

Links:
[1] - https://source.android.com/docs/core/architecture/bootloader/updating#slots

Based-on: https://android-review.googlesource.com/c/platform/external/u-boot/+/1446439
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Guillaume La Roque <glaroque@baylibre.com>
Signed-off-by: Dmitry Rokosov <ddrokosov@salutedevices.com>
Tested-by: Mattijs Korpershoek <mkorpershoek@baylibre.com> # vim3_android
Link: https://lore.kernel.org/r/20241017-android_ab_master-v5-6-43bfcc096d95@salutedevices.com
Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>