crossgcc: Test for m4 and bison

Happened way too often that crossgcc failed
because m4 or bison wasn't installed already.

Change-Id: Ibcca2183edd5db20608015e3898f8fff9a6d11e8
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/1026
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

util/crossgcc/buildgcc

@@ -107,6 +107,9 @@ TAR=`searchgnu tar` || exit $?
 PATCH=`searchgnu patch` || exit $?
 MAKE=`searchgnu make` || exit $?
 
+searchgnu m4 > /dev/null
+searchgnu bison > /dev/null
+
 cleanup()
 {
 	printf "Cleaning up temporary files... "