Turn on -Werror for recovery

Also remove the 0xff comparison when validating the bootloader
message fields. As the fields won't be erased to 0xff after we
remove the MTD support.

Bug: 28202046
Test: The recovery folder compiles for aosp_x86-eng
Change-Id: Ibb30ea1b2b28676fb08c7e92a1e5f7b6ef3247ab
(cherry picked from commit 7aa88748f6ec4e53333d1a15747bc44826ccc410)
diff --git a/bootloader_message/Android.mk b/bootloader_message/Android.mk
index 815ac67..a8c5081 100644
--- a/bootloader_message/Android.mk
+++ b/bootloader_message/Android.mk
@@ -19,6 +19,7 @@
 LOCAL_SRC_FILES := bootloader_message.cpp
 LOCAL_MODULE := libbootloader_message
 LOCAL_STATIC_LIBRARIES := libbase libfs_mgr
+LOCAL_CFLAGS := -Werror
 LOCAL_C_INCLUDES := $(LOCAL_PATH)/include
 LOCAL_EXPORT_C_INCLUDE_DIRS := $(LOCAL_PATH)/include
 include $(BUILD_STATIC_LIBRARY)