Default interpreter to "python" if "python3" doesn't exist

bin/device-install.sh

@@ -1,6 +1,6 @@
 #!/bin/sh
 
-PYTHON=${PYTHON:-python3}
+PYTHON=${PYTHON:-$(which python3 python|head -n 1)}
 
 set -e
 

bin/device-update.sh

@@ -1,6 +1,6 @@
 #!/bin/sh
 
-PYTHON=${PYTHON:-python3}
+PYTHON=${PYTHON:-$(which python3 python|head -n 1)}
 
 # Usage info
 show_help() {