ui/ncurses: Move menu initialisation to cui code

We'll want to re-initialise the petitboot main menu (when the language
is changed), which we can't do if the items are populated entirely from
the main() function.

This change moves the menu initilisation to the cui code, we we can
re-init when necessary.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>