Use hardware.inc from gbdev, or update hardware_constants.asm

[Rangi42]

Right now constants/hardware_constants.asm is very bare-bones, with few constants defined for the values of hardware registers. This means that the code has magic numbers scattered around when setting/checking these values.

We could:

1. Copy constants from pokecrystal's more complete hardware_constants.asm file.
2. Use hardware.inc, which is pretty standard in the GB dev community (see also Use hardware.inc from gbdev pokecrystal#914).
3. Copy some constants from hardware.inc, but use our own file formatting conventions (I'm not a fan of their REGF_VALUE and REGB_VALUE names, or the huge block comments).

In any case, I'd like to sync with the Gen 2 repos on this. (So using hardware.inc directly is probably not the way to go, it would disrupt a lot of existing names.)