Use magiskboot to repack the boot partition

Set TW_INCLUDE_REPACKTOOLS := true

Must also have:
AB_OTA_UPDATER := true

Use magiskboot and provide GUI options to allow users to repack
their existing boot image to install TWRP (or kernels) so we can
stop having to provide installation zips for AB devices. There is
also an option to try to fix a recovery bootloop if the kernel
has been patched to always boot the ramdisk for root, etc.

You will need to pull the below repo into external/magisk-prebuilt
https://github.com/TeamWin/external_magisk-prebuilt

Change-Id: I74196cc6f095a7576d61886dc96cbc18deba9b04

prebuilt/Android.mk

@@ -558,3 +558,12 @@ ifeq ($(TW_INCLUDE_CRYPTO), true)
         endif
     endif
 endif
+
+ifeq ($(TW_INCLUDE_REPACKTOOLS), true)
+    ifeq ($(wildcard external/magisk-prebuilt/Android.mk),)
+        $(warning Magisk repacking tools not found!)
+        $(warning Please place https://github.com/TeamWin/external_magisk-prebuilt)
+        $(warning into external/magisk-prebuilt)
+        $(error magiskboot prebuilts not present; exiting)
+    endif
+endif