Web interface: Add Namecheap dynamic DNS support

The subdomain (or @ for the root domain) goes in the Host Name field.
The domain goes in User Name/Email Address field.
The passkey provided by Namecheap goes in the Password/DDNS Key field.

Wildcard is currently unavailable.
Build: Don't explicitly set the path to KERNELCC/KERNELLD

This way the compiler/linker only need to be in your path, not
located at a specific path in /opt/brcm.