Refactor Kconfig into project/module parts

- Add prompt to MCUBOOT Kconfig and default 'n'
- Add boot/zephyr/Kconfig reference to module.yml
- Relocate module Kconfig into mcuboot/zephyr/
- Relocate module-level Kconfig declarations into the module's
  zephyr/ subdirectory so that it is visible for external build
  configurations.  Removed sourcing of Kconfig.zephyr
- Restore sourcing of Kconfig.zephyr to stripped boot/zephyr/Kconfig.

The MCUBOOT Kconfig seems to control the inclusion of MCUBoot
application code in the Zephyr build.  Because this name matches
the module and an alternate name was used to include MCUBoot's
bootutil package the name usage is a bit unorthodox.

An orthodox and scalable pattern is for a Kconfig matching the
module name to control the exposure of the module itself
(e.g., the public headers).  The first step in this transition is
to make MCUBOOT public and default to 'n'.  Defaulting to 'n' will
prevent this module with interfering with other existing builds.

Adding a reference in module.yml to the boot/zephyr/Kconfig will
define the MCUBOOT symbol before it is used/extended elsewhere.

Incremental verification:

1. (Pass) west build -p always -b nrf52840dk_nrf52840 mcuboot/boot/zephyr
2. (Pass) ./zephyr/scripts/twister --testsuite-root zephyr/tests/subsys/dfu
3. (Pass) ./zephyr/scripts/twister --testsuite-root zephyr/samples/subsys/mgmt/mcumgr/smp_svr/
4. (Pass) ./zephyr/scripts/twister --testsuite-root mcuboot/boot/zephyr
5. (FAIL) ./zephyr/scripts/twister --testsuite-root mcuboot/samples/zephyr_external_config

This new sample is expected to fail while we evolve the minimum
implementation to support it.

Currently the build failure message is:
'''
<command-line>: fatal error: mcuboot-mbedtls-cfg.h: No such file or directory
compilation terminated.
'''

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

Author: legrand-gregshue