Fix example projects

[BlastBrothers]

There are still some issues that I haven't been able to pin down:

• fileio has an issue with remove()
• invaders is very broken. Crashes emulator and hardware. I added some (commented-out) debugging lines to try and isolate the problem - I don't even get that far on emulator, and on hardware it fails on a different line each time. Perhaps this one should just be replaced with a new example?

In addition, sprite, vdu, and invaders really should copy their associated graphics folders to /bin so they can run, but they don't do this currently.

[BlastBrothers]

• Invaders is gone.
• images needed for the examples to run are copied to /bin.
• the issue with remove() is patched over by putting a declaration in fileio.c - I don't know why the compiler isn't picking up the one from stdio but that's a challenge for another day.

[astralaster]

Did some cleanups. Thanks for your contribution!