nds-cmake: new package

[fincs]

Good work. I have two nitpicks:

• On DS, the embedded ROM filesystem isn't called RomFS, it's called NitroFS. Replacing ROMFS with NITROFS everywhere should suffice.
• ExternalProject seems to not integrate well with make clean, and it looks like you had to work around that in your corresponding nds-examples PR. Additionally, it doesn't look like it's possible to access information about targets contained within the ExternalProject, a limitation you also ended up having to work around. I'm still in the process of figuring out the best/cleanest way to support projects that use multiple toolchains/"platforms"/processors in CMake, and that might involve external helper tools or wrapper makefiles. In the interest of making conservative incremental additions, I think the arm7 subproject support (nds_build_arm7 + corresponding changes in nds_create_rom) should be left out for now. However, feel free to leave in the arm7 compiler/linker flag setting logic, as that is ok to have.

[ccawley2011]

OK, done.

[fincs]

PR merged, after being adapted to some refactoring in the cmake machinery that was previously in the works. Thank you very much for your contribution :)