kconfig: clean generated *conf-cfg files

I accidentally dropped '*' in the previous renaming patch.

Revive it so that 'make mrproper' can clean the generated files.

Fixes: d86271af

 ("kconfig: rename generated .*conf-cfg to *conf-cfg")
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>