Will be useful for assembling binaries for the TRS-80 which can't start
at addr 0.

I have yet to test the whole process on real hardware. Soon.

This completes the migration of all Collapse OS code into blkfs!

And adjust rc2014/eeprom recipe

XPACK is more efficient than stripfc was, we can pack more stuff in 8K.
We now have enough space to fit readln.

Big bootstrapping change which greatly reduces the role of the modern
operating system in the bootstrapping process.

More reliable.

It's now a simple flag updated by the C< implementation. No more
routine pointer.

The goal is to avoid tons of "ok" during a LOAD operation.

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.