ANDROID: Automatically perform A/B slot-selection in boot_android.

In order to perform A/B flow as accurately as possible, it should be
triggered from within the boot_android command rather than externally.
This patch makes the slot argument to boot_android optional, and if it
is unspecified, it will be determined automatically via
ab_select_slot().

Bug: 141272741
Test: cuttlefish boots
Change-Id: I5ccafc9fd30d78a794d6d35a2cdd0420f22580d3

Link: https://android-review.googlesource.com/c/platform/external/u-boot/+/1133070/2
Bug: https://baylibre.atlassian.net/browse/RITA-97
Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
2 files changed