Add prompt to MCUBOOT Kconfig and default 'n'

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.

Building with `west build -p always -b nrf52840dk_nrf52840
mcuboot/samples/zephyr_external_config` still fails with:

  error: MCUBOOT (defined at Kconfig.zephyr:682) is assigned in a configuration file, but is not
  directly user-configurable (has no prompt). It gets its value indirectly from other symbols. See
  http://docs.zephyrproject.org/latest/kconfig.html#CONFIG_MCUBOOT and/or look up MCUBOOT in the
  menuconfig/guiconfig interface. The Application Development Primer, Setting Configuration Values,
  and Kconfig - Tips and Best Practices sections of the manual might be helpful too.

  CMake Error at /home/zephyr/mcuboot_dev/zephyr/cmake/modules/kconfig.cmake:326 (message):
    command failed with return code: 1
  Call Stack (most recent call first):
    /home/zephyr/mcuboot_dev/zephyr/cmake/modules/zephyr_default.cmake:108 (include)
    /home/zephyr/mcuboot_dev/zephyr/share/zephyr-package/cmake/ZephyrConfig.cmake:66 (include)
    /home/zephyr/mcuboot_dev/zephyr/share/zephyr-package/cmake/ZephyrConfig.cmake:92 (include_boilerplate)
    CMakeLists.txt:6 (find_package)

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

Author: legrand-gregshue

boot/zephyr/Kconfig

@@ -10,8 +10,8 @@ comment "MCUboot-specific configuration options"
 
 # Hidden option to mark a project as MCUboot
 config MCUBOOT
-	default y
-	bool
+	default n
+	bool  "MCUBoot application"
 	select MPU_ALLOW_FLASH_WRITE if ARM_MPU
 	select USE_DT_CODE_PARTITION if HAS_FLASH_LOAD_OFFSET
 	select MCUBOOT_BOOTUTIL_LIB

boot/zephyr/prj.conf

@@ -1,3 +1,4 @@
+CONFIG_MCUBOOT=y
 CONFIG_DEBUG=y
 CONFIG_PM=n