NB1: Force set boot lun to xbl_a instead of xbl

For some time now, TWRP has suffered from the slot switching
bug which affects only switching from slot B, to slot A. Logs show that
bootcontrol is unable to switch xbl partition. Because in recovery every _a partition
gets a symlink without the slot postfix, gpt utils selects /dev/block/bootdevice/by-name/xbl
instead of /dev/block/bootdevice/by-name/xbl_a when switching to slot A. So force the
gpt-utils library to select only _a/_b variants.

Build the library seperately and replace librecovery_updater_msm with our updated
gpt-utils in order for the bootctrl to notice our change. This should fix the slot
change problem. From limited testing slot switching now works 100% of the time

gpt-utils/gpt-utils.cpp

@@ -667,9 +667,7 @@ int gpt_utils_set_xbl_boot_partition(enum boot_chain chain)
 
         if (chain == BACKUP_BOOT) {
                 boot_lun_id = BOOT_LUN_B_ID;
-                if (!stat(XBL_BACKUP, &st))
-                        boot_dev = XBL_BACKUP;
-                else if (!stat(XBL_AB_SECONDARY, &st))
+                if (!stat(XBL_AB_SECONDARY, &st))
                         boot_dev = XBL_AB_SECONDARY;
                 else {
                         fprintf(stderr, "%s: Failed to locate secondary xbl\n",
@@ -678,9 +676,7 @@ int gpt_utils_set_xbl_boot_partition(enum boot_chain chain)
                 }
         } else if (chain == NORMAL_BOOT) {
                 boot_lun_id = BOOT_LUN_A_ID;
-                if (!stat(XBL_PRIMARY, &st))
-                        boot_dev = XBL_PRIMARY;
-                else if (!stat(XBL_AB_PRIMARY, &st))
+                if (!stat(XBL_AB_PRIMARY, &st))
                         boot_dev = XBL_AB_PRIMARY;
                 else {
                         fprintf(stderr, "%s: Failed to locate primary xbl\n",