build: releasetools: allow the creation of flashable kernels

add a script that will dynamically create a flashable zip based on the fstab and boot image
the zip is also signed, and a hash is outputted to check for integrity
Any external files needed will be automatically compiled (signpack, updater binary)

credits to SlimRoms for the original idea

Change-Id: I823160d4abd8e81eb63d37a5d350d1e075dc7a57

Conflicts:

	core/Makefile
Add more info while flashing

Change-Id: I6ada4cb5b2962073037b28045ca8304d602155c4

Conflicts:

	tools/releasetools/ota_from_target_files