releasetools: Move packages into extras

Instead of removing packages from makefiles,
move them into extras folder in the ota(zip) file.

By default, build/tools/releasetools/extras.txt is used.
If you want to customise it, define this in your BoardConfig:

TARGET_OTA_EXTRAS_FILE := device/VENDORNAME/DEVICENAME/extras.txt

Conflicts:
	core/Makefile
	tools/releasetools/ota_from_target_files

Change-Id: I3d86914ccf262fc02b37d9789e21c1ec0d837701