Revert "build/boot/roms mkUBootRoms: initialise variables"

This reverts commit 7c90a4077f.

causes another build bug. i'm helping someone with the bug now,
i think the workaround for now would be to just use bash, on
this script. until i can figure something better out.

resources/scripts/build/boot/roms_helper

@@ -487,9 +487,6 @@ 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