T1843: run interface-definitions though GCC preprocessor

A lot of XML code is duplicated (VLAN, interface address) for instance. Such
XML definitions should be moved to feature.xml.i files and then just pulled in
via GCC preprocessor #include definition in e.g. bond or ethernet definitions.

This will give us the ability to single-source repeating node definitions as:

• Interface Address
• Interface Description
• Interface Disable
• VLAN (both vif-s and vif-c)

The .in suffix of the interface-definitions is a marker that those files are
input files to the GCC preprocessor. They will be rendered into proper XML
files in the build directory.

Some node definitions have been reworder to remove escaped double quote
occurances which would have been warned about by the GCC preprocessor.