More compatible use of mktemp

This is taken from FILO and slightly enhanced.

Signed-off-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

Change-Id: Ieadd9db3f1013ec1cd9f5a1dc44e17587617f1d1
Original-Change-Id: I961a7ddcd39657c9463806d7b82757eff0a4ac57
Original-Signed-off-by: Patrick Georgi <patrick.georgi@secunet.com>
Original-Reviewed-on: http://review.coreboot.org/190
Reviewed-on: https://review.coreboot.org/14386
Tested-by: build bot (Jenkins)
Reviewed-by: Martin Roth <martinroth@google.com>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

util/xcompile/xcompile

@@ -51,7 +51,7 @@ clean_up() {
 }
 
 # Create temporary file(s).
-TMPFILE="$(mktemp /tmp/temp.XXXX 2>/dev/null || echo /tmp/temp.78gOIUGz)"
+TMPFILE="$(mktemp /tmp/temp.XXXXXX 2>/dev/null || echo /tmp/temp.coreboot.$RANDOM)"
 touch "$TMPFILE"
 trap clean_up EXIT