construct flightmode menu

def flightmode_menu():
    '''construct flightmode menu'''
    global flightmodes
    ret = []
    idx = 0
    for (mode,t1,t2) in flightmodes:
        modestr = "%s %us" % (mode, (t2-t1))
        ret.append(MPMenuCheckbox(modestr, modestr, 'mode-%u' % idx))
        idx += 1
        mestate.flightmode_selections.append(False)
    return ret