Inav: Arming failure when killswitch is set on alternate position of remote controller arm switch

[biocage]

Current Behavior

Set a two-position switch so that the LOW position is mode KILLSWITCH and the HIGH position is mode ARM. Transition to mode ARM. The system will fail to arm, and will display a generic error messages on the OSD:

UNABLE TO ARM
DISABLE ARM SWITCH FIRST

Expected behavior

The device would either arm, or provide a succinct message indicating why it failed to arm.

Version:

INAV/MATEKF722 2.2.0 Jun 3 2019 / 14:05:06 (f59251378)

GCC-7.3.1 20180622 (release) [ARM/embedded-7-branch revision 261907]

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[digitalentity]

Such setup doesn't make much sense, but this is what happens:

1. Arming switch is off, killswitch is on
2. Arming switch is on, killswitch is off
   
   
   
   • Arm blocking code checks arming switch first and since at this point killswitch is not cleared yet, sets the DISABLE ARM SWITCH FIRST blocker.
   
   
   • Arm blocking code disables killswitch blocker, but at this point arming is already blocked
   
   

The linked PR changes the order to make sure all other blockers are checked before the arming switch to avoid this.