Buildsystem improvements

[vifino]

While I don't want cmake or anything of that sort, writing better Makefiles is something that should be done.

• [x] Get rid of the multiple makefiles, merge them.
• [x] Support cross-compiling.
• [ ] Support different dynamic library file extensions.
• [ ] Split modules in portable/unportable sections.
• [ ] CI for 3ds port.
• [ ] CI for card10 port.

[vifino]

Thanks to @20kdc, a majority of this is done! :tada: