kbuild: do not build mconf & lxdialog unless needed

Due to a limitation in kbuild all objects referred
by xxx-y or xxx-objs will be build when one of
the targets needs to e build.

This caused lxdialog to be build pulling in ncurses
that is not always available.
So avoid building mconf & lxdialog unless really needed.

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