[baro_board] make configure work for BARO_PERIODIC_FREQUENCY (needed to use define before)
[sys_time] maximum of 16 sys_time timers by default (was 8)

actually makes the baro periodic timer (for BARO_BOARD) usable by default on fixedwings
[ins] alt_float: if USE_BARO_BOARD, dt is 1/BARO_PERIODIC_FREQUENCY