Add custom kBuild footer to perform extra post-processing.

Post-processing is not possible otherwise since kBuild doesn't provide
any hooks that would let us do something after all makefiles are read in
but before targets start getting executed.
Add recognition of _PP_SOURCES in install to perform preprocessing.

Preprocessing is done using a python script, as in the original build env.
Allow LocalConfig.kmk define things before Config.kmk takes part and add perl/python defs.
Generate cached build settings file.

This gets included from Config.kmk and contains variables which require
calling external tools and perform text processing to get their values.
This speeds up the build process and makes it more clean.
Add generation of application.ini.