Use metadata for build targets

It has been in the wishlists of several developers for years, the ability to compile directories as targets or even use tags as targets themselves. I prefer the latter.

The idea is to tag ReactOS components with simple tags like "usermode", "kernelmode", "driver", etc.. and have them as targets so that it would be possible to make certain sections of reactos without the pain of compiling everything else.

So for example instead of doing make ntoskrnl_clean | hal_clean | win32k_clean
a simple make kernelmode_clean would suffice, or a psdk change could be simply tested with make usermode, or make applications etc..

Maybe a special flag/command may be used to tell rbuild to scan the rbuild files and create a makefile for these targets. The actual implementation of this enhancement is beyond my abilities...

CORE-2327 added metadata support in rbuild files, maybe this could lead the way for this enhancement.