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

AndroidBoot.mk

@@ -13,10 +13,12 @@ else
   SIGNED_KERNEL := SIGNED_KERNEL=0
 endif
 
+ifeq ($(BOOTLOADER_PLATFORM),)
+  BOOTLOADER_PLATFORM := $(TARGET_BOARD_PLATFORM)
+endif
+
 ifeq ($(TARGET_BOARD_PLATFORM),msm8660)
   BOOTLOADER_PLATFORM := msm8660_surf
-else
-  BOOTLOADER_PLATFORM := $(TARGET_BOARD_PLATFORM)
 endif
 
 ABOOT_OUT := $(TARGET_OUT_INTERMEDIATES)/ABOOT_OBJ