recovery: Move bldrmsg offset symbols to bootloader_message.cpp

* bootloader_message.cpp is the only file using
  BOOTLOADER_MESSAGE_OFFSET_IN_MISC and WIPE_PACKAGE_OFFSET_IN_MISC,
  so we can move their definitions to the cpp.
  This prevents the need to set BOARD_RECOVERY_BLDRMSG_OFFSET
  in every module that includes the header.

* Global cflags are no longer supported Oreo and up,
  so set the BOARD_RECOVERY_BLDRMSG_OFFSET via make variable

* Simplify logic, always set BOARD_RECOVERY_BLDRMSG_OFFSET.

Change-Id: I2b902bcce7f5ca13472e0ac30ac01b4991294dbe
diff --git a/bootloader_message_twrp/Android.mk b/bootloader_message_twrp/Android.mk
index e7a3ea0..932cd04 100644
--- a/bootloader_message_twrp/Android.mk
+++ b/bootloader_message_twrp/Android.mk
@@ -31,5 +31,10 @@
 ifeq ($(TW_IGNORE_MISC_WIPE_DATA), true)
     LOCAL_CFLAGS += -DIGNORE_MISC_WIPE_DATA
 endif
+ifeq ($(BOOTLOADER_MESSAGE_OFFSET),)
+    LOCAL_CFLAGS += -DBOARD_RECOVERY_BLDRMSG_OFFSET=0
+else
+    LOCAL_CFLAGS += -DBOARD_RECOVERY_BLDRMSG_OFFSET=$(BOOTLOADER_MESSAGE_OFFSET)
+endif
 LOCAL_EXPORT_C_INCLUDE_DIRS := $(LOCAL_PATH)/include
 include $(BUILD_SHARED_LIBRARY)