add checkpatch config

- do not lint the submodule
- do not lint the grgsm code
- do not lint headers, checkpatch does not understand that this is a c++
project and any change to existing headers will cause issues.
- also do not do arcane C varargs checks that do not apply to C++

Change-Id: Ie7a9fbd021e12a88db30212240af2332c6cdcb37

.checkpatch.conf

@@ -0,0 +1,4 @@
+--exclude osmocom-bb/.*
+--exclude .*h
+--exclude Transceiver52M/grgsm_vitac/.*
+--ignore FUNCTION_WITHOUT_ARGS