mgmt: mcumgr: Consolidate Kconfig into a single file

For some reason, MCUMgr used to consist of two separate Kconfig
files, likely due to the fact that the author's original intention was
to split the "management" and "mcumgr-specific" options in order to
integrate additional libraries. In practice this never worked out, so
coalesce them into a single file.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>