kconfig: Fix kconfig type determination

Board specific Kconfigs must be parsed manually.
Previously we just assumed they were bool types,
but for more complex builds this is not enough.
This adds more complete Kconfig parsing to read
the type for these configs.

Change-Id: I61d086752c06abb9ea1edcd6851495e954234542
Reviewed-on: https://pigweed-review.googlesource.com/c/zephyr/zephyr-bazel/+/405573