Add example of how to use the DSi AES engine

[AntonioND]

No description provided.

[mdigiorgio]

Adding RSA usage example is addressed by #182

[mdigiorgio]

RSA usage example is now merged.

For what concerns AES, an investigation is needed to:

• understand what the possible use cases can be
• estimate the effort
• understand what is provided for AES encryption/decryption. It seems like we only have access to registers (https://github.com/blocksds/libnds/blob/master/include/nds/arm7/aes.h) and there are no BIOS functions

[profi200]

A working superset of this AES engine is implemented in this old code. I had been planning to rewrite this and include it in libn3ds again.
https://github.com/derrekr/fastboot3DS/blob/master/source/arm9/hardware/crypto.c

Since i wrote the entire file i can relicense as needed.

[asiekierka]

A good chunk of the code there concerns keyX/keyY, which I don't think we have a particular intention of handling.

I think I have a decent understanding of how the AES engine works, but we could really use a discussion w/r/t on-demand DMA/NDMA/timer allocation/acquisition - as proper (= efficient) use of the AES engine requires borrowing not one, but two NDMA channels ;-)