feat: make TARGETS optional so it can be overridden

[DanielSRS]

Issue

Currently all architectures are built by default (a520051), but the way the TARGETS variable is defined, it doesn't allow it to be overridden by projects using this as a submodule, like rawdrawandroidexample does.

Trying to override the default targets wont work. Seems like it's just get concatenated with the default ones.

Solution

Make TARGETS optional so it can be overridden while keep the default value building all architectures by default.

[DanielSRS]

I am not that much familiar with make, so there is chances this is not needed at all and I just don't know how to do what I wanted 😅

[cnlohr]

No, if you potentially want to override it that's exactly how you do it.

good job.