SHMUP: Restructure Makefile

Continuing the discussion in https://github.com/gbdev/gb-asm-tutorial/pull/57:

@ISSOtm said:

Regarding the Makefile, I have a few primary suggestions right now. Most of them revolve around making the Makefile drastically simpler, following the theme of introducing new concepts incrementally.

- [ ] Remove the `SRCDIR` variable. The point of variables like `OBJDIR` is to allow building out-of-tree in a flexible location; however, it makes imo no sense to allow pointing the Makefile at a *different* source tree, since it's inherently part of it.
- [ ] Use `${}` for variables instead of `$()`. This helps differentiate them from function calls (which only accept the latter syntax); this is especially useful when both kinds are nested.
- [ ] Remove all of the `*DIR` and `RGB*` variables. For a "directed" project like this, it should be fine to hardcode all paths; this will notably remove one layer of indirection, and allow the reader to familiarise themselves with what that layer of indirection will "resolve" to. (There may be a case for keeping all of the `RGB*` variables, in case someone doesn't have RGBDS in their path, or under different names? Idk.)
- [ ] Build in-tree. Building out-of-tree is better for a lot of reasons, but adds significant complexity (creating output directories, doing more path substitution instead of just extension rewriting, etc.). Again, since this is a "controlled environment", it should be fine to do things slightly dirty if it helps introducing concepts gradually; additionally, I believe experiencing the downsides of in-tree builds should help justify the extra complexity of out-of-tree builds when they're introduced later.