fs/ubi: add option to use custom ubinize config file

By default, the UBI FS target creates a ubinize configuration
file on-the-fly, for a single volume.  Add an option to specify
a custom config file.

E.g., one might want to deploy a system with a volume for the
built ubifs image, a volume for a future upgrade image, and a
volume for user data.

Signed-off-by: Danomi Manchego <danomimanchego123@gmail.com>
Reviewed-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>