build system: improve portability by not relying on extraordinary dd options

Change-Id: Ibc7fe38ae11e9d477f85fdcf760b27abc6013f94
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: http://review.coreboot.org/9791
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
diff --git a/Makefile.inc b/Makefile.inc
index 204e518..9ef29d6 100644
--- a/Makefile.inc
+++ b/Makefile.inc
@@ -560,7 +560,7 @@
 	@printf "    CBFS       $(subst $(obj)/,,$(@))\n"
 # The full ROM may be larger than the CBFS part, so create an empty
 # file (filled with \377 = 0xff) and copy the CBFS image over it.
-	tr '\000' '\377' < /dev/zero 2> /dev/null | dd of=$@.tmp bs=8192 iflag=fullblock count=$$(($(CONFIG_ROM_SIZE) / 8192)) 2> /dev/null
+	dd if=/dev/zero bs=$(CONFIG_ROM_SIZE) count=1 | tr '\000' '\377' > $@.tmp
 	dd if=$(obj)/coreboot.pre of=$@.tmp bs=8192 conv=notrunc 2> /dev/null
 	$(CBFSTOOL) $@.tmp add-stage -f $(objcbfs)/ramstage.elf -n $(CONFIG_CBFS_PREFIX)/ramstage -c $(CBFS_COMPRESS_FLAG)
 ifeq ($(CONFIG_PAYLOAD_NONE),y)