mgmt/mcumgr: Split Kconfig into smaller Kconfigs

The commit splits the main mcumgr Kconfigs into primary Kconfig
that collect general mcumgr options for Zephyr (Zephyr specific
commands, transports, and so on), into lib/Kconfig
that collects library options and command groups' Kconfigs.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>