Documentation/kbuild: Document COMPILE_TEST dependencies

limits FOO to module (=m) or disabled (=n).

Compile-testing

~~~~~~~~~~~~~~~

If a config symbol has a dependency, but the code controlled by the config

symbol can still be compiled if the dependency is not met, it is encouraged to

increase build coverage by adding an "|| COMPILE_TEST" clause to the

dependency. This is especially useful for drivers for more exotic hardware, as

it allows continuous-integration systems to compile-test the code on a more

common system, and detect bugs that way.

Note that compile-tested code should avoid crashing when run on a system where

the dependency is not met.

Kconfig recursive dependency limitations

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~