kernel: add own Kconfig for OpenWrt mtdsplit drivers

There are pretty many OpenWrt patches against mtd subsystem resulting
in a bit of mess and growing maintenance cost.
My idea is to use an extra "mtdsplit" directory with OpenWrt specific
files (including Kconfig).
This is the first step to achieve this. This patch adds a "mtdsplit"
directory with Kconfig and replaces 4 patches with a single one.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>

SVN-Revision: 42287