cbfstool: Add -mno-ms-bitfields on (mingw)

The default gcc on mingw will process the __attribute__ ((packed)) in
a different way other than non-win system.

Change-Id: Iac9f4476c922472d0b447f1c3ef60e8e13bd902f
Signed-off-by: Zheng Bao <zheng.bao@amd.com>
Signed-off-by: Zheng Bao <fishbaozi@gmail.com>
Reviewed-on: http://review.coreboot.org/1603
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>

util/cbfstool/Makefile.inc

@@ -15,6 +15,10 @@ cbfsobj += cbfstool.o
 
 CBFSTOOLFLAGS=-DCOMPACT -g
 
+ifeq ($(shell uname -s | cut -c-7 2>/dev/null), MINGW32)
+CBFSTOOLFLAGS+=-mno-ms-bitfields
+endif
+
 $(objutil)/cbfstool:
 	mkdir -p $@