AndroidBoot: Allow gcc toolchain override

In some cases, we dont want to reply on Android build system variables
to define gcc toolchain version. Allow for gcc toolchain override.

Change-Id: Ia1ed75408975db5920529baa87b9d811121d955e
diff --git a/AndroidBoot.mk b/AndroidBoot.mk
index 1b336e3..713863b 100644
--- a/AndroidBoot.mk
+++ b/AndroidBoot.mk
@@ -1,10 +1,14 @@
 #Android makefile to build lk bootloader as a part of Android Build
 
-ifndef 2ND_TARGET_GCC_VERSION
+ifeq ($(BOOTLOADER_GCC_VERSION),)
+ifndef $(2ND_TARGET_GCC_VERSION)
 CROSS_COMPILE := ../../../prebuilts/gcc/linux-x86/arm/arm-eabi-$(TARGET_GCC_VERSION)/bin/arm-eabi-
 else
 CROSS_COMPILE := ../../../prebuilts/gcc/linux-x86/arm/arm-eabi-$(2ND_TARGET_GCC_VERSION)/bin/arm-eabi-
 endif
+else # BOOTLOADER_GCC_VERSION defined
+CROSS_COMPILE := ../../../prebuilts/gcc/linux-x86/arm/$(BOOTLOADER_GCC_VERSION)/bin/arm-eabi-
+endif
 
 # Set flags if we need to include security libs
 ifeq ($(TARGET_BOOTIMG_SIGNED),true)