kernel/3.10: move OpenWrt MTD options into a submenu

Separate OpenWrt specific MTD options from the mainline
option by moving those into a new submenu in the kernel
configuration interface.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@37997 3c298f89-4303-0410-b956-a3cf2f4a3e73