Allow specifying kernel toolchain and prefix

Change-Id: Ib75cfe772e073f2196455f3e3dfba247f6b3feff

Conflicts:
	core/tasks/kernel.mk
recovery: build uncompressed ramdisk

Change-Id: I6027868864ec2696f20705faf4c68c684e7ef459
Signed-off-by: Chirayu Desai <chirayudesai1@gmail.com>

Conflicts:
	core/Makefile
build: don't clear recovery ramdisk

Clearing recovery ramdisk removes all files place there by the
device tree and all busybox symlinks placed there by recovery

Change-Id: I19e6164856a01a83a6e2c50ef96742edcb9f570f
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