mcuboot: move the MCUboot kconfig file to modules/

After the MCUboot configuration options were moved out of the main
Kconfig.zephyr they were placed in a new Kconfig.mcuboot in the root of
the repo. This is not right, and the file belongs in the modules/
folder.

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