tools: Fix mingw tools build

mkenvimage does not build due to missed os_support.o and unsupported
file modes S_IRGRP S_IWGRP.
Tested with mingw 4.2.1 on ubuntu 12.04.

Signed-off-by: Vladimir Yakovlev <nagos@inbox.ru>
diff --git a/tools/Makefile b/tools/Makefile
index 8097d95..a7d1e18 100644
--- a/tools/Makefile
+++ b/tools/Makefile
@@ -196,7 +196,8 @@
 	$(HOSTCC) $(HOSTCFLAGS) $(HOSTLDFLAGS) -o $@ $^
 	$(HOSTSTRIP) $@
 
-$(obj)mkenvimage$(SFX):	$(obj)crc32.o $(obj)mkenvimage.o
+$(obj)mkenvimage$(SFX):	$(obj)crc32.o $(obj)mkenvimage.o \
+	$(obj)os_support.o
 	$(HOSTCC) $(HOSTCFLAGS) $(HOSTLDFLAGS) -o $@ $^
 	$(HOSTSTRIP) $@