Relocate module Kconfig beneath $module/zephyr/

Separate `boot/zephyr/Kconfig` into the portion to be
picked up through `zephyr/module.yml` (variable declarations)
and the portion appropriate for a top-level Kconfig
(main menu, sourcing Kconfig.zephyr).  Relocate all the
module-specific Kconfig content beneath `$mcuboot/zephyr/`
and extend `$mcuboot/zephyr/module.yml` to reference it.

Verified by:
1. building with `-t guiconfig` and manually verifying the
   relocation of mcuboot Kconfig content within the Modules:mcuboot
   section.
2. building a full image and verifying the .map file did not change
   in flash or ram size.

Signed-off-by: Gregory SHUE <[email protected]>

Author: legrand-gregshue