ncurses: add support for 256 colors

[Thomas: fixup nano to use $(NCURSES_CONFIG_SCRIPTS) now that the
ncurses config script can have a different name depending on the
configuration.]

Signed-off-by: Thierry Bultel <tbultel@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>