From: Sam Ravnborg <sam@ravnborg.org>

When building a kernel right after 'make mrproper' resulted in a very short
run, and no sign that .config was missing.  This has been fixed by adding a
new rule for .config in the top-level Makefile, and a new target
'silentoldconfig' in scripts/kconfig/Makefile.

Cleaned up a bit in scripts/kconfig/Makefile