ui/ncurses: Always provide a key definition for backtab

ui/ncurses: Always provide a key definition for backtab

Petitboot environments will probably want a basic terminfo defintion
(eg, vt220) rather than a full linux or xterm, but vt220 and friends
don't define a backtab key. Backtab can be useful for proper form
navigation, and without a key definition, we just get an escape, which
exits the current screen.

This change provides a static definition for KEY_BTAB, so we should
always have one available.

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