kconfig/menuconfig: do not hardcode '.config'

Export and use the function conf_get_configname()
to retreive the default configuration filename.

Suggested by: Roman Zippel <zippel@linux-m68k.org>

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

diff --cc scripts/kconfig/lkc.h
Simple merge

diff --cc scripts/kconfig/mconf.c
Simple merge