build/boot/roms mkUBootRoms: initialise variables

they were outside the scope, outside of the if statements.

in some shells, this is ok.

we use "sh" so the user could have any shell.

be a bit nicer to the more asininely technically
correct sh implementations out there

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

resources/scripts/build/boot/roms_helper

@@ -487,6 +487,9 @@ mkUbootRom() {
 	target_uboot_config="${3}"
 	cbfstool_path="${4}"
 
+	target_ubootelf=""
+	target_ubdir=""
+
 	if [ "${target_uboot_config}" = "default" ]; then
 		target_ubdir="payload/u-boot/${board}"
 	else