um: cleanup Kconfig files

We can handle all not architecture specific UM configuration directly in
the newly added arch/um/Kconfig.  Do so by merging the Kconfig.common,
Kconfig.rest and Kconfig.um files into arch/um/Kconfig, and move the main
UML menu as well.

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