kernel: Allow setting dtbo target path prefix

gs101 uses DTBO_OBJ/arch/arm64/boot/dts/google/dtbo.img
vs DTBO_OBJ/arch/arm/boot/dtbo.img on previous devices.

Change-Id: I945d31cb093fae97385cf84c273088ee7a4edf0b
diff --git a/config/BoardConfigKernel.mk b/config/BoardConfigKernel.mk
index 6111a9b..53cadfa 100644
--- a/config/BoardConfigKernel.mk
+++ b/config/BoardConfigKernel.mk
@@ -31,6 +31,8 @@
 #                                        makefile. Can be overriden in device trees
 #                                        in the event of prebuilt kernel.
 #
+#   TARGET_KERNEL_DTBO_PREFIX          = Override path prefix of TARGET_KERNEL_DTBO.
+#                                        Defaults to empty
 #   TARGET_KERNEL_DTBO                 = Name of the kernel Makefile target that
 #                                        generates dtbo.img. Defaults to dtbo.img
 #
@@ -146,8 +148,9 @@
 
 # Set DTBO image locations so the build system knows to build them
 ifeq (true,$(filter true, $(TARGET_NEEDS_DTBOIMAGE) $(BOARD_KERNEL_SEPARATED_DTBO)))
+TARGET_KERNEL_DTBO_PREFIX ?=
 TARGET_KERNEL_DTBO ?= dtbo.img
-BOARD_PREBUILT_DTBOIMAGE ?= $(TARGET_OUT_INTERMEDIATES)/DTBO_OBJ/arch/$(KERNEL_ARCH)/boot/$(TARGET_KERNEL_DTBO)
+BOARD_PREBUILT_DTBOIMAGE ?= $(TARGET_OUT_INTERMEDIATES)/DTBO_OBJ/arch/$(KERNEL_ARCH)/boot/$(TARGET_KERNEL_DTBO_PREFIX)$(TARGET_KERNEL_DTBO)
 endif
 
 # Set use the full path to the make command