buildgcc: Break if parts of the toolchain are missing

We test for the presence of a couple of tools and even print an error.
But the tool didn't stop there.

Change-Id: I40dcf7894408ea7b24d5f68c76df4b7541f469bd
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/111
Tested-by: build bot (Jenkins)
Reviewed-by: Marc Jones <marcj303@gmail.com>

util/crossgcc/buildgcc

@@ -88,9 +88,9 @@ searchgnu()
 	exit 1
 }
 
-TAR=`searchgnu tar`
-PATCH=`searchgnu patch`
-MAKE=`searchgnu make`
+TAR=`searchgnu tar` || exit $?
+PATCH=`searchgnu patch` || exit $?
+MAKE=`searchgnu make` || exit $?
 
 cleanup()
 {