Separate mtdparts command from jffs2

Currently the mtdparts commands are included in the jffs2 command support.
This doesn't make sense anymore since other commands (e.g. UBI) use this
infrastructure as well now. This patch separates the mtdparts commands from
the jffs2 commands making it possible to only select mtdparts when no JFFS2
support is needed.

Signed-off-by: Stefan Roese <sr@denx.de>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>