Stack fixes

This round squeezes another K of RAM out of a few safe places. With the MAVLink app restructuring we should be finally then down again to a reasonable amount of RAM. This PR is needed to allow RC config via USB, currently broken on master.

Stack usage while armed:
```
Processes: 21 total, 3 running, 18 sleeping
CPU usage: 62.85% tasks, 0.75% sched, 36.40% idle
Uptime: 16.351s total, 6.506s idle

 PID COMMAND                   CPU(ms) CPU(%)  USED/STACK PRIO(BASE) STATE 
   0 Idle Task                    6507 36.404     0/    0   0 (  0)  READY 
   1 hpwork                        576  3.895   740/ 2040 192 (192)  w:sig 
   2 lpwork                         72  0.449  1148/ 2040  50 ( 50)  w:sig 
   3 init                          436  0.000  1404/ 3496 100 (100)  w:sem 
  50 nshterm                         1  0.000   492/ 1192 240 (100)  w:sem 
 113 sdlog2_writer                1380 12.808   940/ 2040  60 ( 60)  w:sem 
  52 commander                     124  0.299  1980/ 2944 215 (215)  w:sig 
  53 commander_low_prio              4  0.000   604/ 2896  50 ( 50)  w:sem 
  55 px4io                         765  4.868   916/ 1992 240 (240)  w:sem 
  60 mavlink_if0                   185  1.198  1028/ 1944 100 (100)  w:sig 
  61 mavlink_rcv_if0                 7  0.000   788/ 2896 215 (215)  w:sem 
  76 sensors_task                  597  3.820  1180/ 1992 250 (250)  w:sem 
 100 attitude_estimator_ekf       3184 21.498 12916/13992 250 (250)  w:sem 
  80 sdlog2                        275  1.123  1868/ 2992  70 ( 70)  w:sig 
  83 gps                            62  0.224   988/ 1992 220 (220)  w:sig 
 102 position_estimator_inav       377  2.771  2300/ 3992 250 (250)  w:sem 
 104 mc_att_control                580  3.820  1148/ 1992 250 (250)  w:sem 
 106 mc_pos_control                 96  0.823  1036/ 1992 250 (250)  w:sem 
 109 dataman                       703  0.000   692/ 1992 250 (250)  w:sem 
 111 navigator                      81  0.449   860/ 1992 250 (250)  w:sem 
 118 top                           250  4.794  1252/ 1696 100 (100)  RUN   
```

Stack usage with USB link:

```
Processes: 21 total, 3 running, 18 sleeping
CPU usage: 57.83% tasks, 0.67% sched, 41.50% idle
Uptime: 96.332s total, 36.902s idle

 PID COMMAND                   CPU(ms) CPU(%)  USED/STACK PRIO(BASE) STATE 
   0 Idle Task                   36905 41.499     0/    0   0 (  0)  READY 
   1 hpwork                       3661  3.711   740/ 2040 192 (192)  w:sem 
   2 lpwork                        460  0.593  1148/ 2040  50 ( 50)  w:sig 
   3 init                          438  0.000  1404/ 3496 100 (100)  w:sem 
 139 top                           177  4.008  1252/ 1696 100 (100)  RUN   
  52 commander                     380  0.222  1980/ 2944 215 (215)  w:sig 
  53 commander_low_prio             32  0.000   604/ 2896  50 ( 50)  w:sem 
  55 px4io                        4740  4.825   916/ 1992 240 (240)  w:sem 
  60 mavlink_if0                  1155  1.262  1028/ 1944 100 (100)  READY 
  61 mavlink_rcv_if0                46  0.074   788/ 2896 215 (215)  w:sem 
  76 sensors_task                 3709  3.860  1180/ 1992 250 (250)  w:sem 
 100 attitude_estimator_ekf      20380 21.455 12916/13992 250 (250)  w:sem 
  80 sdlog2                       1078  0.222  1868/ 2992  70 ( 70)  w:sig 
  83 gps                           360  0.371   988/ 1992 220 (220)  w:sem 
 102 position_estimator_inav      2628  2.821  2300/ 3992 250 (250)  w:sem 
 104 mc_att_control               3690  4.008  1148/ 1992 250 (250)  w:sem 
 106 mc_pos_control                772  0.816  1036/ 1992 250 (250)  w:sem 
 109 dataman                       703  0.000   692/ 1992 250 (250)  w:sem 
 111 navigator                     460  0.445   860/ 1992 250 (250)  w:sem 
 121 mavlink_if1                   773  9.131   940/ 1944 100 (100)  w:sig 
 122 mavlink_rcv_if1                 1  0.000   788/ 2896 215 (215)  w:sem 
```