feat(params): add board-level read-only parameter support

Integrators can declare read-only parameters in a per-board YAML file:
readonly_params.yaml.

There are two ways to define the read-only params:
- "block": default writable, explicitly list params to be locked
- "allow": default readonly, explicitly list params to be writable

Enforcement is activated by `param lock` in rcS after all startup
scripts have run, so board defaults and airframe scripts can still set
params during init.

The feedback via MAVLink uses the new
MAV_PARAM_ERROR_READ_ONLY as part of the PARAM_ERROR message.

ROMFS/px4fmu_common/init.d/rcS

@@ -651,6 +651,10 @@ else
 		. ${R}etc/init.d/rc.autostart.post
 	fi
 
+	#
+	# Lock read-only parameters (if configured for this board).
+	#
+	param lock
 
 	set BOARD_BOOTLOADER_UPGRADE ${R}etc/init.d/rc.board_bootloader_upgrade
 	if [ -f $BOARD_BOOTLOADER_UPGRADE ]