mkimage: fix argument parsing on BSD systems

The getopt(3) optstring '-' is a GNU extension which is not available on BSD
systems like OS X.

Remove this dependency by implementing argument parsing in another way. This
will also change the lately introduced '-b' switch behaviour.

Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/Makefile b/Makefile
index f03fec1..293fad0 100644
--- a/Makefile
+++ b/Makefile
@@ -898,7 +898,7 @@
 MKIMAGEFLAGS_u-boot.img = -f auto -A $(ARCH) -T firmware -C none -O u-boot \
 	-a $(CONFIG_SYS_TEXT_BASE) -e $(CONFIG_SYS_UBOOT_START) \
 	-n "U-Boot $(UBOOTRELEASE) for $(BOARD) board" -E \
-	-b $(patsubst %,arch/$(ARCH)/dts/%.dtb,$(subst ",,$(CONFIG_OF_LIST)))
+	$(patsubst %,-b arch/$(ARCH)/dts/%.dtb,$(subst ",,$(CONFIG_OF_LIST)))
 else
 MKIMAGEFLAGS_u-boot.img = -A $(ARCH) -T firmware -C none -O u-boot \
 	-a $(CONFIG_SYS_TEXT_BASE) -e $(CONFIG_SYS_UBOOT_START) \