AndroidBoot.mk: Allow NAND and eMMC variants to be built independently

Due to legacy reasons we always built NAND variant along with eMMC
variant.  Update to exposes top level rules to allow building just
NAND or eMMC variants as TARGET_NAND_BOOTLOADER and
TARGET_EMMC_BOOTLOADER respectively.

Targets using TARGET_BOOTLOADER continue to get both variants for
older targets that use it and is planned to be removed in future.

Change-Id: Ic86c7335c013f6d7dc7c30812dceb88d2ab54c08
6 files changed