Feature: support custom 'defines'

Feature: support custom 'defines'

Some libraries or packages have custom defines, such as the firewall _en_firewall_debug_.

Provide simple documentation of the defines, in the data/build/defines file.
Provide build settings values for specifying these custom defines.
Provide parameter for manually overriding defines.

Some path settings have been renamed to avoid confusion (such as '--c_path' is now '--path_c').

Enable _en_firewall_debug_ by default.