make: AndroidBoot: Set cross toolchain path for lk

Update the LK build system to use TARGET_GCC_VERSION for
setting cross compile toolchain path.

Change-Id: Ib797ba8a8ce9615a36a98a716887cd2ce8363541
diff --git a/AndroidBoot.mk b/AndroidBoot.mk
index 6637405..c85677e 100644
--- a/AndroidBoot.mk
+++ b/AndroidBoot.mk
@@ -29,7 +29,7 @@
 # ELF binary for ABOOT
 TARGET_ABOOT_ELF := $(PRODUCT_OUT)/aboot.elf
 $(TARGET_ABOOT_ELF): ABOOT_CLEAN | $(ABOOT_OUT)
-	$(MAKE) -C bootable/bootloader/lk BOOTLOADER_OUT=../../../$(ABOOT_OUT) $(BOOTLOADER_PLATFORM) $(EMMC_BOOT) $(SIGNED_KERNEL)
+	$(MAKE) -C bootable/bootloader/lk TOOLCHAIN_PREFIX=$(CROSS_COMPILE) BOOTLOADER_OUT=../../../$(ABOOT_OUT) $(BOOTLOADER_PLATFORM) $(EMMC_BOOT) $(SIGNED_KERNEL)
 
 # NAND variant output
 TARGET_NAND_BOOTLOADER := $(PRODUCT_OUT)/appsboot.mbn