makefile: Enable defining BOOTLOADER_TARGET elsewhere
makefile: Enable defining BOOTLOADER_TARGET elsewhere

Currently BOOTLOADER_PLATFORM is assumed to be same as
TARGET_BOARD_PLATFORM. Remove this dependency when
BOOTLOADER_PLATFORM is defined elsewhere. This would
enable same binary to be compiled for multiple lunch
options by enabling definition of BOOTLAODER_PLATFORM
in places like BoardConfig.mk files or other parts of
the android build system.

Change-Id: I55d63c826bf299399e23d59a931b0a03a4859449

efidroid/lk,CanarySolutions/little-kernel,Foxda-Tech/polaris-bootable-bootloader-lk,Foxda-Tech/polaris-bootable-bootloader-lk,efidroid/lk,efidroid/lk,efidroid/lk,CanarySolutions/little-kernel,CanarySolutions/little-kernel,CanarySolutions/little-kernel,Foxda-Tech/polaris-bootable-bootloader-lk,efidroid/lk,CanarySolutions/little-kernel,Foxda-Tech/polaris-bootable-bootloader-lk,Foxda-Tech/polaris-bootable-bootloader-lk