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.

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

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.

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

It seems that I had forgotten in an earlier commit.

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

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...

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.

as well as emulator support for it. We can now write an edited
block back to "blkfs".

Saves quite a few bytes in the final binary.

It's more hassle than it seems to move the stable ABI...

I forgot to do it earlier. NOP having an opcode of 0, it didn't have
any effect.

Should be significantly faster now. Less push/popping and usage of
DE instead of IY.

Saves quite a bit of binary space. More than 300 bytes in forth1.bin

Remove flagsToBC boot routine.

Makes boot binary a bit bigger, but that "_pdacc" word will be
reused in high level apps.

It's not used

The former was only used in the peculiar context of "/emul". The
regular case is actually HERE pointing to RAMEND on boot.

This allows optional subsystems to not needlessly reserve too much
system memory and it also reduces offsets bookkeeping.

Let's revisit the relinking approach, with LITA this time.
This reverts commit 6652125d47.