dist/tools/kconfiglib: add configuration evaluation function

[leandrolanzieri]

Contribution description

This removes the existing hack, in which we wrote the configuration to /dev/null to trigger its evaluation. Instead a function for this specific purpose is added. Besides being cleaner, it avoids issues like the one reported at https://forum.riot-os.org/t/kconfig-issues-in-riot/3646.

Testing procedure

• To verify that configuration issues are still being caught, you can apply an invalid one like the following in the kconfig test app, and run make:

diff --git a/tests/kconfig/Kconfig b/tests/kconfig/Kconfig
index c18dffdd8f..cbfec4b519 100644
--- a/tests/kconfig/Kconfig
+++ b/tests/kconfig/Kconfig
@@ -20,3 +20,8 @@ config APP_MSG_2
         Activate printing the second message
 
 endmenu # Application
+
+config FOO_VALUE
+    int "Foo"
+    range 1 3
+    default 4

• Green CI

Issues/PRs references

Forum post https://forum.riot-os.org/t/kconfig-issues-in-riot/3646

[MrKevinWeiss]

ACK, even though it is using a private function the kconfig library seems stable enough...