Revert "add support for building flashable kernel zips"

This reverts commit 1ec1e21928d2c47376cdb6241038a431a264a50f.
releasetools: bring back boot.img handling

This is required for galaxysmtd and other devices with custom
boot format.

This partially reverts "releasetools: Properly handle boot images that use dt"

Change-Id: Ib35001c2d9b906067e5a3c84712dc3a283a38cbb

Conflicts:
	core/Makefile
	tools/releasetools/common.py