[NES II] Run NES game image from DOS filesystem

[JMarlin]

Description

With the read-only filesystem basically working and an ability to load and run large binaries from disk, the next logical step is to slap some NES cart payloads onto the disk along with a little loader binary that can do the loading and jump into the cart -- just like we were doing with raw disk NES games, but pulling from the FS instead (so ostensibly we should be able to put multiple games on the disk)

My general idea is to extract the binary file loading from the run command and into its own os function that the loader can also use. Then we can have the loader, two 16k PRG files, and an 8k CHR file on the disk. The PRG files we can just make self loading so that the loader stub literally just has to effectively do a "load mario_a.prg" and it'll put itself into the right area.

A tricky thing here is that we have to use 100% of the cart SRAM to store the game code, so this is also going to need to involve making sure we can load all of NOS and the loader stub into the RAM available in the NES. Which is like 768 bytes or so. (2k - 0.25k ZP - 0.25k stack - 0.25k sector buffer - 0.25k twos buffer - 0.25k text buffer = 2k - 5(0.25k) = 2k - 1.25k = 0.75k). Might be able to get away with stealing the text buffer to get a full 1k.

Acceptance Criteria

• [x] Test actually running a binary using the run command
• [x] Move NOS load position to just above the text buffer, make sure it can fit into ~3 sectors and run properly
• [x] Extract binary load code into NOS itself, update run command to call into it
• [x] Write script to transform an iNES into two PRG binaries with 0x8000 load address and one CHR binary
• [x] Create loader binary that can load first PRG, swap cart SRAM mapping, load second PRG, read CHR into CHR RAM, and finally switch into cart mode and jump to the reset vector

[JMarlin]

Ideas for moving around NOS to fit this scheme:

• Move all the floppy.asm stuff onto the ROM
• Reduce the boot sector code to only include the actual 'OS routines'
• Update the boot sector code to automatically binload and jump into a SHELL file after it's done being loaded
• Move the command processor into SHELL -- it can live at $9000 and be replaced with no issue when the NES loader stub starts itself

[JMarlin]

Moved the floppy.asm stuff, tested with the nos_romemu mode and it appears to work okay so far. Still over by 300-something bytes, but I have hope that we can get rid of that by ditching the command processor.

[JMarlin]

Move NOS load position to just above the text buffer, make sure it can fit into ~3 sectors and run properly

Moved it, made sure it fits, haven't tested though so I can't mark this complete yet until I can test with the shell binary.

[JMarlin]

NOS has been split into the system and the shell and is confirmed to run properly from disk