samba36: add package with hotplug.d script for auto sharing

samba36: add package with hotplug.d script for auto sharing

The new samba36-hotplug package provides a hotplug.d script for the
"mount" subsystem. It automatically shares every mounted block device.

It works by updating /var/run/config/samba file which:
1) Is read by procd init script
2) Gets wiped on reboot providing a consistent state
3) Can be safely updated without flash wearing or conflicting with user
   changes being made in /etc/config/samba

Cc: Rosy Song <rosysong@rosinson.com>
Cc: Jo-Philipp Wich <jo@mein.io>
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>