Makefile: Add clean_builtsrc, clean_builtassets

[Dragorn421]

TLDR

This PR adds clean_builtsrc (to rebuild src/),
and clean_builtassets (to rebuild assets/)
make clean && make : 3min40s
make clean_builtsrc && make : 1min40s
make clean_builtassets && make : 2min10s

Motivation

make clean is often used when building (make) fails for some obscure reason, or the ROM doesn't build OK for equally obscure reasons

Building from clean (make after make clean) takes a long time, 3min40s for me without -j.
The "clean" operation can however be separated into two parts: cleaning built assets (in build/assets) and built source (in build/src)
It is very rarely useful or required to rebuild all of both of these, and typically rebuilding all of either is enough

This PR adds clean_builtsrc and clean_builtassets alongside clean.
Where clean removes the whole build folder of intermediate files, clean_builtsrc focuses on build/src and clean_builtassets on build/assets (they both also remove other things clean removes, see the Makefile)

Building from clean_builtsrc (i.e. re-building src/) is 1min40s for me, half the time to build from clean (still make without -j)
And building from clean_builtassets (i.e. re-building assets/) is 2min10s for me

[AngheloAlf]

What is this find doing exactly?

[Dragorn421]

It's removing files directly inside build/, i.e.
dmadata_table_spec.h ldscript.txt spec undefined_syms.txt z64.map (in build/)

I could also put that list of files into the makefile tbh but it felt less random to remove all files

[Dragorn421]

(fwiw this is not advanced find usage at all, assuming this may get in and if we keep it I'll add a comment on it though)

[hensldm]

I'm fine with this implementation, but thoughts on instead of new targets, use clean with an env variable (i.e. clean TARGET=builtsrc). I'll admit does seem a little more clunky.

[hensldm]

Suggested change

	clean_builtassets:
	builtassetsclean:

Wonder if this would be better to mirror assetclean? (same idea with clean_builtsrc)
Or perhaps builtassets_clean and assets_clean?