Backend: fix apex mounts on fastboot boot'd TWRP

- work around different /apex environment causing loop mounts to fail on TWRP only when it's initiated by `fastboot boot twrp.img`

META-INF/com/google/android/update-binary

@@ -52,6 +52,7 @@ mount_apex() {
   [ -d /system_root/system/apex ] || return 1;
   local apex dest loop minorx num var;
   setup_mountpoint /apex;
+  $BB mount -t tmpfs tmpfs /apex -o mode=755 && $BB touch /apex/apextmp;
   minorx=1;
   [ -e /dev/block/loop1 ] && minorx=$($BB ls -l /dev/block/loop1 | $BB awk '{ print $6 }');
   num=0;
@@ -89,6 +90,7 @@ mount_apex() {
   $($BB grep -o 'export .* /.*' /system_root/init.environ.rc | $BB sed 's; /;=/;'); unset export;
 }
 umount_apex() {
+  [ -d /apex ] || return 1;
   local dest loop var;
   for var in $($BB grep -o 'export .* /.*' /system_root/init.environ.rc 2>/dev/null | $BB awk '{ print $2 }'); do
     if [ "$(eval echo \$OLD_$var)" ]; then
@@ -103,6 +105,7 @@ umount_apex() {
     $BB umount -l $dest;
     [ "$loop" ] && $BB losetup -d $loop;
   done;
+  [ -f /apex/apextmp ] && $BB umount /apex;
   $BB rm -rf /apex 2>/dev/null;
 }
 mount_all() {