Always having to import md32_dis.py into a Python interpreter just to
disassemble one instruction got kind of annoying, so I added some basic
command-line functionality to the module.

The BROM log length gets set to zero when the log is either dumped out
the UART or read by one of the DL mode "read BROM log" commands
(0xDD/0xDF), so we only limit the the output length when the reported
length is non-zero. When the reported length is zero, we simply print
each character until we either hit the default max length (0x400 bytes,
the length of the BROM log buffer) or encounter a non-printable
character.

The obfuscated version isn't terribly useful, and it can always be
regenerated by performing the XOR again, so let's just save the
deobfuscated version by default.

This lets us set it to negative values without having to do the two's
complement manually.

IMEM only has enough space for 512 instruction words, so make sure we
don't accidentally try to write more than that.

Loading a 16-bit immediate only works when the destination register is
r0-r7. To enable this function to set any one of the 32 general-purpose
registers, convert this into a load-shift-add of 11-bit and 5-bit
immediates.

Unfortunately, while "sethi" can be approximated with a 16-bit imediate
load and a shift by immediate, there's no way to then OR the destination
register with a 16-bit immediate. This means if we want to set a
register without using any temporary registers, we need to load the
16-bit immediate, shift the destination left by 4, add the next nybble
to the destination, and then repeat that shift-by-4-and-add-nybble
sequence another three times to finish the 32-bit load.

It's currently hardcoded to work with the MT8163, but it shouldn't take
much effort to get it working with other SoCs.

These don't really have much to do with the ISA RE.

Some of these don't discuss ISA stuff, but I'll just add them all for
now and remove the irrelevant ones later.

Certain values can be written using only one instruction, so do that
when possible.

It's currently hardcoded to work with the MT8163, but it shouldn't take
much effort to get it working with other SoCs.

Make sure there's data in the echo data buffer before checking any bytes
in it.

usbdl: throw an error if not handshaked

Add MT6753 support

This seems to give better results than just the mnemonic and arg format
alone.

After some testing, I found that the PCM reset vector is zero, so the
NOP sled isn't needed. I'm not really sure why my code wasn't working
without it before, but it does now, so let's remove it. Also, change the
instructions to register writes so it's easier to tell that it's
working.