posix-configs/rpi: Fix mavlink over UART (/dev/ttyAMA0)

UART is the primary interface for telemetry radio: https://docs.emlid.com/navio2/ardupilot/hardware-setup/#uart-radio

Check first if /dev/ttyUSB0 exists (https://docs.emlid.com/navio2/ardupilot/hardware-setup/#usb-radio); if not, fall back to configuring over UART (/dev/ttyAMA0).

Use stricter check for character special file (-c) rather than just file (-f).

'console=serial0,115200' needs to be removed from /cmdline.txt as additional configuration step. This should be documented in the Navio2 section of docs.px4.io. Presumably, this is already performed as part of the Raspberry Pi OS prebuilt image Emlid spins.

posix-configs/rpi/px4.config

@@ -45,9 +45,12 @@ mc_rate_control start
 
 mavlink start -x -u 14556 -r 1000000
 
-if [ -f /dev/ttyUSB0 ]
+if [ -c /dev/ttyUSB0 ]
 then
 	mavlink start -x -d /dev/ttyUSB0
+elif [ -c /dev/ttyAMA0 ]
+then
+	mavlink start -x -d /dev/ttyAMA0
 fi
 
 navio_sysfs_rc_in start

posix-configs/rpi/px4_fw.config

@@ -44,9 +44,12 @@ fw_pos_control_l1 start
 
 mavlink start -x -u 14556 -r 1000000
 
-if [ -f /dev/ttyUSB0 ]
+if [ -c /dev/ttyUSB0 ]
 then
 	mavlink start -x -d /dev/ttyUSB0
+elif [ -c /dev/ttyAMA0 ]
+then
+	mavlink start -x -d /dev/ttyAMA0
 fi
 
 navio_sysfs_rc_in start

posix-configs/rpi/px4_test.config

@@ -44,9 +44,12 @@ mc_att_control start
 
 mavlink start -x -u 14556 -r 1000000
 
-if [ -f /dev/ttyUSB0 ]
+if [ -c /dev/ttyUSB0 ]
 then
 	mavlink start -x -d /dev/ttyUSB0
+elif [ -c /dev/ttyAMA0 ]
+then
+	mavlink start -x -d /dev/ttyAMA0
 fi
 
 navio_sysfs_rc_in start