sitl: add generic way to override params via ENV variables

ROMFS/px4fmu_common/init.d-posix/rcS

@@ -126,6 +126,15 @@ then
 	set AUTOCNF yes
 fi
 
+# Allow overriding parameters via env variables: export PX4_PARAM_{name}={value}
+env | while IFS='=' read -r line; do
+  value=${line#*=}
+  name=${line%%=*}
+  case $name in
+    "PX4_PARAM_"*) param set "${name#PX4_PARAM_}" "$value" ;;
+  esac
+done
+
 # multi-instance setup
 # shellcheck disable=SC2154
 param set MAV_SYS_ID $((px4_instance+1))