releasetools: add squisher hook

Change-Id: Id90cea9f1db10a6979419d05e1b6905647da2534
Ignore LOCAL_AAPT_INCLUDE_ALL_RESOURCES on mdpi & ldpi devices

Ensures that certain packages don't include unneeded hdpi or higher
assets.

Change-Id: I8e62486f21382e21ec8a587ce96b2383761aad3a
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

Change-Id: I3d86914ccf262fc02b37d9789e21c1ec0d837701

Conflicts:
	core/Makefile
	tools/releasetools/ota_from_target_files