luci-mod-network: Add PXE Boot options tab under DHCP and DNS

This commit adds support for managing `boot` sections in /etc/config/dhcp.

Signed-off-by: Paul Dee <systemcrash@users.noreply.github.com>
[Hide alias interfaces in network select, reword commit message]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>