MAINTAINERS: New kconfig maintainer

Yann has been the de facto maintainer of kconfig for some time. Update
the KCONFIG entry with his emails address and git tree.

Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Michal Marek <mmarek@suse.cz>