The SD card driver is a bit too slow to be bearable. I'll write
_crc16 in z80 and see how it goes.

This will allow us to read from blkfs in tests.

Less noisy this way

This is done by adding a "C<* override" layer that is reset to 0 on
ABORT.

The problem was that when ABORT happened during a LOAD, we had a
resetted RSP and started from a clean INTERPRET, but LOAD didn't
have the opportunity to restore C<*, which caused it to continue
interpreting from the faulty BLK.

With a C<* override, we don't need to *restore* C<*, we just need to
clear the override.

This is more than cosmetic, it's also highly usable. The presence
or absence of the "ok" message allows us to know whether the command
aborted. Previously, the "> " prompt appeared when the system expected
a prompt in the INTERPRET context, whether the previous command aborted
or not.

Also, this allows us to get rid of that ugly FLAGS global variable.

That becomes irrelevant in Forth

The way is clear for complete stage1 bootstrapping on the RC2014
target!

We can get rid of acia.z80, but not of acia.fs yet, we still need it.

It didn't work for values over 0xff. Duh!

It doesn't serve much purpose and takes up many precious bytes. And,
after all, parsed didn't do it either...

2DROP was left in boot.z80 because it's used in icore.

It didn't have z80a anymore.

This allows us to remove forth/z80a.fs. Another big step towards
self-hosting!

It seems that I had forgotten in an earlier commit.

I don't remove z80a.fs yet because build process needs it, but that
will change shortly.

As long as our target was the first word of the "user" dict, using
target's prev to compute offset was fine, but when the target is not
the first word, this system breaks down.

This is the case when, instead of including source code in our boot
binary, we paste it in Collapse OS' prompt.

Also, adjust RC2014 recipe to include stage 3 assembling instructions
with the "paste into prompt" method.

In addition to making the driver significantly faster, it allows us
to make SDC port configuration opaque to sdc.fs.

This has been tested in the RC2014 emulator. I could edit blocks with
the Block Editor and they would correctly be written to SDC.

"0 LIST" in the RC2014 emulator w/ SD card works!

It's not possible to reliably determine its end and with some memory
initialization scenarios, it makes RLDICT fail.

I can read a block from blkfs in RC2014 emulator. It even checks
CRC16 successfully!

Goes through initialization in RC2014 emulator.

As I wrote in my "Clarify signed-ness" commit, "0 <" is broken.

Also, made this unit a bit more compact. The RC2014 stage1 can
really use some breathing room...

stripfc requires 2 spaces before "(" to strip a comment. This was
a low hanging fruit for me to get stage1.bin under the 8K line.

That thing is getting tight...

All of that should help me implement sdc support much faster in
Forth.

I now get to "fls" in recipes/rc2014/sdcard!

so far, it goes through initialization with the rc2014/sdcard recipe.

Also, fix some PSP leaks related to LOAD.