Add_Flags not sufficient for all cases of path spesific flags. Override and remove flags functions needed.

[GiorgiaLine]

The Add_Flags functionality for path specific command line flags is not sufficient for all uses of path specific flags. In some cases path specific flags overwrite or remove flags in the 'generic' list and functionality to support this is needed.

[t00sa]

This is probably part of a wider refactor of the build_config module.

[hiker]

I think #313 should address this. Afaik, 'override' functionality is given by the order of parameters for any compiler - later arguments override earlier ones (-g -O0 -O3).

The important bit would be to combine the two type of flags Fab supports (generic and path-specific: a generic flag is just a path-specific flag which matches anything :) ).