bcp38: add init script

bcp38: add init script

This commit adds a simple procd init script for bcp38 with the sole purpose
to register a configuration change trigger for /etc/config/bcp38.

The change will allow for automatic firewall reloads triggered by invoking
/sbin/reload_config or through ubus config change events emitted by LuCI.

With the init script in place and started, calling

  ubus call service event '{"type":"config.change","data":{"package":"bcp38"}}'

or

  /sbin/reload_config

will issue an /etc/init.d/firewall reload if /etc/config/bcp38 has been
modified since the last reload_config call.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>