NuttX boards reduce CONFIG_NFILE_DESCRIPTORS 54 -> 20

[dagar]

Waiting on #12123.

Once logger is updated we can safely decrease the maximum number of file descriptors which will free up a considerable amount of ram (~ 10 kB).

 PID COMMAND                   CPU(ms) CPU(%)  USED/STACK PRIO(BASE) STATE FD
   0 Idle Task                  217510 42.700   288/ 1024   0 (  0)  READY  3
   1 hpwork                       4050  0.800   740/ 1780 249 (249)  w:sig  4
   2 lpwork                        557  1.700   976/ 1780  50 ( 50)  w:sig  8
   3 init                         1670  0.000  1864/ 2844 100 (100)  w:sem  3
   4 wq:manager                      4  0.000   824/ 1172 244 (244)  w:sem  7
  15 dataman                       150  0.300   744/ 1180  90 ( 90)  w:sem  4
  18 wq:lp_default                 277  0.000   740/ 1500 205 (205)  w:sem  7
  20 wq:I2C1                      2566  0.600   776/ 1244 248 (248)  w:sem  7
  25 wq:hp_default                1282  0.200   708/ 1500 243 (243)  w:sem  7
 125 wq:I2C2                     10755  2.200   648/ 1244 247 (247)  w:sem  7
 134 wq:SPI2                     96200 19.000   820/ 1396 253 (253)  PEND   7
 157 sensors                     17425  3.700  1456/ 1964 238 (238)  w:sem 10
 159 commander                    6112  1.100  1680/ 3212 140 (140)  w:sig  6
 160 commander_low_prio             15  0.000   568/ 2996  50 ( 50)  w:sem  6
 190 gps                          1563  0.300  1072/ 1516 209 (209)  w:sem  4
 214 mavlink_if0                 10469  2.100  1784/ 2484 100 (100)  w:sig  4
 215 mavlink_rcv_if0              3066  0.600  1520/ 2836 175 (175)  w:sem  4
 231 fmu                         14763  2.900   968/ 1324 241 (241)  w:sem  8
 290 ekf2                        66577 11.200  4536/ 6572 239 (239)  w:sem  4
 292 mc_att_control              16152  3.100  1000/ 1660 240 (240)  w:sem  4
 296 mc_pos_control               7730  0.300  1224/ 1860 239 (239)  w:sem 11
 300 navigator                    1541  0.300  1232/ 1764 105 (105)  READY  4
 316 log_writer_file              3315  0.800   592/ 1164  60 ( 60)  w:sem  4
 314 logger                      14055  3.299  3072/ 3644 234 (234)  RUN    4

Processes: 24 total, 4 running, 20 sleeping, max FDs: 20
CPU usage: 54.50% tasks, 2.80% sched, 42.70% idle
DMA Memory: 5120 total, 1536 used 1536 peak
Uptime: 511.210s total, 217.511s idle

The sensors module and FlightTasks still need to be moved to the updated uORB::Subscription (or uORB::SubscriptionBlocking #12371). After that's done we can decrease CONFIG_NFILE_DESCRIPTORS again to 10 or so.

[dagar]

@RomanBapst this should help alleviate memory issues on F4 boards.