rom.sh: copy tmprom to TMPDIR for modification

don't rely on build/coreboot.rom staying in place,
because sometimes it can get purged under certain
conditions, due to idiosyncrasies in the coreboot
build system, even when we don't explicitly clean it

Signed-off-by: Leah Rowe <leah@libreboot.org>

include/rom.sh

@@ -80,7 +80,9 @@ mkcorebootbin()
 {
 	[ "$target" = "$tree" ] && return 0
 
-	tmprom="$srcdir/build/coreboot.rom"
+	tmprom="$TMPDIR/coreboot.rom"
+	mv "$srcdir/build/coreboot.rom" "$tmprom" || $err "!mktmprom"
+
 	initmode="${defconfig##*/}"; displaymode="${initmode##*_}"
 	initmode="${initmode%%_*}"
 	[ -n "$displaymode" ] && displaymode="_$displaymode"