base-files: allow non-standard rootfs volume name in UBI in sysupgrade

This commit allows to use non-standard UBI volume name as the rootfs
volume in sysupgrade.

ex.:
  The U-Boot on Buffalo WXR-2533DHP checks existence and checksum of
  "ubi_rootfs" volume when booting, so this name is required.

OpenWrt currently provides several patches:

490-ubi-auto-attach-mtd-device-named-ubi-or-data-on-boot.patch
491-ubi-auto-create-ubiblock-device-for-rootfs.patch
492-try-auto-mounting-ubi0-rootfs-in-init-do_mounts.c.patch

to facilitate ubi rootfs automount. However the upstream kernel
also supports the means of booting from a fully custom ubi
partition name and ubi volume name via bootargs/kernel's cmdline
parameters:

ubi.mtd=mtd_partition_name
ubi.block=rootfs_volume_name
root=/dev/ubiblock$X_$Y

For more information and examples visit the wiki over at linux-mtd:
<http://www.linux-mtd.infradead.org/faq/ubifs.html>
<http://www.linux-mtd.infradead.org/doc/ubi.html

>

Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com> [reworded commit]