build: target_config_check: make check more lenient

Always allow options set to builtin (=y) when modular setting (=m) is
expected. This can happen when a package is added explicitly (in a
target defintion or site.mk) that is also pulled in as a dependency of
another builtin package.

Fixes: 9e23534e ("build: rework config generation")
Fixes: #2046
(cherry picked from commit 8b64517f)