tools: env: Add support for direct read/write UBI volumes

tools: env: Add support for direct read/write UBI volumes

Up to now we were able to read/write environment data from/to UBI
volumes only indirectly by gluebi driver. This driver creates NAND MTD
on top of UBI volumes, which is quite a workaroung for this use case.

Add support for direct read/write UBI volumes in order to not use
obsolete gluebi driver.

Forward-ported from this patch:
http://patchwork.ozlabs.org/patch/619305/

Original patch:
Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>
Forward port:
Signed-off-by: S. Lockwood-Childs <sjl@vctlabs.com>