um: create a proper drivers Kconfig

Merge arch/um/Kconfig.char and arch/um/Kconfig.net into a new
arch/um/drivers/Kconfig.  This fits the way Kconfig files are placed
elsewhere in the kernel tree.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>