cpu: create an empty file when no microcode files are given

Having an empty microcode file makes it more easy to debug
in comparison to a not existing file in cbfs. There are some
platforms (e.g. ep80579) which support microcode updates but
not having any microcode updates yet in our tree.

These platform hang the build because `cat` is called with no
parameters.

Change-Id: I2699bde0c62ae62ca888686f8b496e845c36d970
Signed-off-by: Alexander Couzens <lynxis@fe80.eu>
Reviewed-on: http://review.coreboot.org/12109
Tested-by: build bot (Jenkins)
Reviewed-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
diff --git a/src/cpu/Makefile.inc b/src/cpu/Makefile.inc
index d939061..52b58f7 100644
--- a/src/cpu/Makefile.inc
+++ b/src/cpu/Makefile.inc
@@ -40,7 +40,7 @@
 $(obj)/cpu_microcode_blob.bin: $$(cpu_microcode_bins)
 	@printf "    MICROCODE  $(subst $(obj)/,,$(@))\n"
 	@echo $(cpu_microcode_bins)
-	cat $+ > $@
+	cat /dev/null $+ > $@
 
 cbfs-files-$(cbfs_include_ucode) += cpu_microcode_blob.bin
 cpu_microcode_blob.bin-file := $(cpu_ucode_cbfs_file)