fs/ubi: expose BINARIES_DIR to ubinize.cfg

fs/ubi: expose BINARIES_DIR to ubinize.cfg

It is often necessary to refer to other images that are present in
BINARIES_DIR from a custom ubinize configuration e.g. to include the
kernel in a UBI volume.

As we do for BR2_ROOTFS_UBIFS_PATH, replace BINARIES_DIR when copying
the file.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
[yann.morin.1998@free.fr:
  - add the blurb in the help text
  - rewrap commit log
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>