Add system to send messages from the ARM7 to the ARM9

[AntonioND]

Right now there is no way to send debug print messages from the ARM7 to the ARM9.

The idea is to have a function on the ARM9 like consoleArm7Init(PrintConsole *console, size_t buffer_size) that allocates a shared buffer in main RAM and passes it to the ARM7. Then, the ARM7 can write to it and send a FIFO message to the ARM9 when it finishes writing a line, for example. Or maybe the ARM9 can check the buffer regularly and print it. The idea is that the ARM9 would immediately know that all text obtained from the ARM7 needs to be redirected to a specific PrintConsole.

On the ARM7 side, there would be a bool that is set to true whenever the console is initialized. We would also need a very small printf (there isn't a lot of space on the ARM7). Maybe a custom version that only supports %c, %d, %x and %s.

[AntonioND]

Also, we need something like this but for asserts.

[AntonioND]

Prototype here:
blocksds/libnds#148
#246

[AntonioND]

So the console part of this is ready (the two PRs have been merged), only the assertions are left.

[AntonioND]

I've created a new task for the asserts because this work will be quite different from the console messages. #247