dnsmasq: support tftp_unique_root in /etc/config/dhcp

dnsmasq: support tftp_unique_root in /etc/config/dhcp

The TFTP server provided by dnsmasq supports serving a select boot image
based on the client's MAC or IP address. This allows an administrator
to activate this feature in /etc/config/dhcp. Here is an example
/etc/config/dhcp that configures dnsmasq with --tftp-unique-root=mac:

...

config dnsmasq
	option enable_tftp 1
	option tftp_root /usr/libexec/tftpboot
	option tftp_unique_root mac

config boot router
	option serveraddress 192.168.1.1
	option servername tftp.example.com
	option filename openwrt-initramfs-kernel.bin

...

With this configuration, dnsmasq will serve
/usr/libexec/tftpboot/00-11-22-33-44-55/openwrt-initramfs-kernel.bin to
the client with MAC address 00:11:22:33:44:55.

Signed-off-by: W. Michael Petullo <mike@flyn.org>