kconfig: refactor ncurses package checks for building mconf and nconf
The mconf (or its infrastructure, lxdiaglog) depends on the ncurses.
Move and rename check-lxdialog.sh to mconf-cfg.sh to make it work in
the same way as for qconf and gconf.
This commit fixes some more weirdnesses.
The nconf also needs ncurses packages. HOSTLOADLIBES_nconf is set
to the libraries needed for nconf, but the cflags is not explicitly
set. Actually, nconf relies on the check-lxdialog.sh for the proper
cflags: