kconfig: do not hardcode ".config" filename

Rather than hardcoding ".config" use conf_get_configname(), which also
respects the environment variable KCONFIG_CONFIG.

This fixes "make silentoldconfig" when KCONFIG_CONFIG is used and also
suggests the given filename for "Load" and "Save as" in qconf.

Signed-off-by: Markus Heidelberg <markus.heidelberg@web.de>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>