The U-Boot script embedded in the GL.iNet QSDK factory image hardcodes
the GL-B3000 flash layout. Replace the UBI offset and size with
placeholders filled in by the gl-qsdk-factory recipe, so other GL.iNet
boards with a different layout can use the same script.
The script is copied under a per-device name, which keeps the FIT
description unchanged and avoids devices overwriting each other's
substituted copy in KDIR_TMP.
Signed-off-by: Gilberto Conde <gilbertorconde@gmail.com>