soong_config: Add BOOTLOADER_MESSAGE_OFFSET

* libbootloader_message now uses Android.bp

Change-Id: I750a04be97dbe01564d45cd605744b15f209e408
diff --git a/build/soong/soong_config.mk b/build/soong/soong_config.mk
index 32e34eb..1444d0b 100644
--- a/build/soong/soong_config.mk
+++ b/build/soong/soong_config.mk
@@ -1,10 +1,12 @@
 add_json_str_omitempty = $(if $(strip $(2)),$(call add_json_str, $(1), $(2)))
+add_json_val_default = $(call add_json_val, $(1), $(if $(strip $(2)), $(2), $(3)))
 
 _contents := $(_contents)    "Lineage":{$(newline)
 
 # See build/core/soong_config.mk for the add_json_* functions you can use here.
 $(call add_json_str_omitempty, Additional_gralloc_10_usage_bits, $(TARGET_ADDITIONAL_GRALLOC_10_USAGE_BITS))
 $(call add_json_bool, Apply_msm8974_1440p_egl_workaround, $(filter true,$(TARGET_MSM8974_1440P_EGL_WORKAROUND)))
+$(call add_json_val_default, Bootloader_message_offset, $(BOOTLOADER_MESSAGE_OFFSET), 0)
 $(call add_json_bool, Has_legacy_camera_hal1, $(filter true,$(TARGET_HAS_LEGACY_CAMERA_HAL1)))
 $(call add_json_str, Java_Source_Overlays, $(JAVA_SOURCE_OVERLAYS))
 $(call add_json_bool, Should_skip_waiting_for_qsee, $(filter true,$(TARGET_KEYMASTER_SKIP_WAITING_FOR_QSEE)))