Add unpack_bootimg to otatools to modify boot.img

unpack_bootimg in conjunction with mkbootimg can be used to re-create
boot.img with a different kernel image and ramdisk.

Change-Id: I9615facc9335885989772a0dd7f08217e2143a45
diff --git a/core/Makefile b/core/Makefile
index c650022..67d5d3f 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -3778,6 +3778,7 @@
   simg2img \
   sload_f2fs \
   tune2fs \
+  unpack_bootimg \
   update_host_simulator \
   validate_target_files \
   verity_signer \