cmd: Add 'ab_select' command

For A/B system update support the Android boot process requires to send
'androidboot.slot_suffix' parameter as a command line argument. This
patch implementes 'ab_select' command which allows us to obtain current
slot by processing the A/B metadata.

The patch was extracted from commit [1] with one modification: the
separator for specifying the name of metadata partition was changed
from ';' to '#', because ';' is used for commands separation.

[1] https://android-review.googlesource.com/c/platform/external/u-boot/+/729880/2

Signed-off-by: Ruslan Trofymenko <ruslan.trofymenko@linaro.org>
Signed-off-by: Igor Opaniuk <igor.opaniuk@gmail.com>
Reviewed-by: Alistair Strachan <astrachan@google.com>
Reviewed-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/cmd/Makefile b/cmd/Makefile
index 0aa3741..43a6b0e 100644
--- a/cmd/Makefile
+++ b/cmd/Makefile
@@ -12,6 +12,7 @@
 
 # command
 obj-$(CONFIG_CMD_AES) += aes.o
+obj-$(CONFIG_CMD_AB_SELECT) += ab_select.o
 obj-$(CONFIG_CMD_ADC) += adc.o
 obj-$(CONFIG_CMD_ARMFLASH) += armflash.o
 obj-y += blk_common.o