build/boot/roms: bugfix: unitialised variable

When using e.g. -p grub in build/boot/roms, it will
error out. This patch fixes that.

E.g.

./build boot roms t440pmrc_12mb -p grub

Seldom used feature and it was overlooked. Most people
won't use the option that triggered the error.

resources/scripts/build/boot/roms

@@ -29,6 +29,7 @@
 set -u -e
 
 projectname="$(cat projectname)"
+opts=""
 
 listboards() {
 	for boarddir in resources/coreboot/*; do