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:

HOST_EXTRACFLAGS += $(shell $(CONFIG_SHELL) $(check-lxdialog) -ccflags) \
                    -DLOCALE

The code above passes the ncurses flags to all objects, even for conf,
qconf, gconf.  Let's pass the ncurses flags only to mconf and nconf.

Currently, the presence of ncurses is not checked for nconf.  Let's
show a prompt like the mconf case.

According to Randy's report, the shell scripts still need to carry
the fallback code in case the pkg-config fails to find the ncurses
packages.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>