mk: oldconf: Fix an undefined 'config_h' variable warning
mk: oldconf: Fix an undefined 'config_h' variable warning

The warning was introduced in e51a50a ("mk: oldconf: Remove processing
of option.conf from scripts / macros") and merged through #750.

vrxfile/embox-trik,abusalimov/embox,Kefir0192/embox,gzoom13/embox,mike2390/embox,Kakadu/embox,gzoom13/embox,vrxfile/embox-trik,Kakadu/embox,vrxfile/embox-trik,gzoom13/embox,abusalimov/embox,Kefir0192/embox,Kefir0192/embox,vrxfile/embox-trik,gzoom13/embox,embox/embox,Kakadu/embox,abusalimov/embox,Kakadu/embox,embox/embox,gzoom13/embox,abusalimov/embox,embox/embox,abusalimov/embox,Kefir0192/embox,mike2390/embox,mike2390/embox,abusalimov/embox,gzoom13/embox,vrxfile/embox-trik,Kakadu/embox,Kakadu/embox,Kefir0192/embox,vrxfile/embox-trik,vrxfile/embox-trik,mike2390/embox,embox/embox,Kefir0192/embox,embox/embox,Kakadu/embox,mike2390/embox,gzoom13/embox,Kefir0192/embox,embox/embox,mike2390/embox,mike2390/embox