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 <[email protected]>
Reviewed-by: Simon Glass <[email protected]>
Tested-by: Guillaume La Roque <[email protected]>
Signed-off-by: Dmitry Rokosov <[email protected]>
Tested-by: Mattijs Korpershoek <[email protected]> # vim3_android
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Mattijs Korpershoek <[email protected]>

boot/android_ab.c

@@ -52,7 +52,7 @@ static int ab_control_default(struct bootloader_control *abc)
 	if (!abc)
 		return -EFAULT;
 
-	memcpy(abc->slot_suffix, "a\0\0\0", 4);
+	memcpy(abc->slot_suffix, "_a\0\0", 4);
 	abc->magic = BOOT_CTRL_MAGIC;
 	abc->version = BOOT_CTRL_VERSION;
 	abc->nb_slot = NUM_SLOTS;
@@ -328,7 +328,8 @@ int ab_select_slot(struct blk_desc *dev_desc, struct disk_partition *part_info,
 		 * or the device tree.
 		 */
 		memset(slot_suffix, 0, sizeof(slot_suffix));
-		slot_suffix[0] = BOOT_SLOT_NAME(slot);
+		slot_suffix[0] = '_';
+		slot_suffix[1] = BOOT_SLOT_NAME(slot);
 		if (memcmp(abc->slot_suffix, slot_suffix,
 			   sizeof(slot_suffix))) {
 			memcpy(abc->slot_suffix, slot_suffix,