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.

and remove cfspack, which will not ever be used again.

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

They changed in my earlier commit.

Saves quite a few bytes in the final binary.

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

Now that we use xcomp unit, boot entry creation can be a bit
cleaner.

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