This patch adds the interface-name option for each dhcp config
in /etc/config/dhcp.

With the interface_name option users can define a DNS name for each dhcp section
that will be resolved by dnsmasq with the underlaying interface address.

For example:
config dhcp 'lan'
	option interface 'lan'
	...
	list interface_name 'home.lan'
	...

Signed-off-by: Daniel Danzberger <daniel@dd-wrt.com>
Signed-off-by: Hans Dedecker <dedeckeh@gmail.com> [PKG_RELEASE increase]