add support for custom releasetools

Conflicts:
	core/Makefile
Redo BOARD_CUSTOM_BOOTIMG_MK support

Simplified version of the previous implementation. Recovery's ramdisk
is spun off from the main recovery target again to allow overriding
just the image-generation step
build: Handle custom boot images properly

When a pre-built image should be used, it should be stored in the
target files zip so that it can be used with external signing
processes.

Original-Change-Id: I2661af9ac58af30bb9314b552775046d3abf44e0
Change-Id: I10b3bd0bb33489b8ffb26d16d002f8dd6ff405ad
Remove HomeSample from builds

As good as it looks, we already have a launcher (Launcher3)

Change-Id: I87cac70eec30fd62d6f7ecb7777caa9d240fe0ee