Add two load chunks:
 - First chunk checks system (if it's XL and memory is ok).
 - Second chunk contains the contents for the shadow memory beneath the
   ROM and copies the contents to their destination address.
   It also moves the screen memory to low memory in front of the program.
Some small first changes for the 'atarixl' target.
"save area" is used to save modified system variables and ports. It's
shared between the second and main chunk.
Code & data for "system check" load chunk in the atarixl target.
Code & data for "shadow RAM preparation" load chunk in the atarixl target.
Intermediate check-in, unfinished...
move ZPSAVE into area already saved by 2nd load chunk
rename CIOV and SIOV to CIOV_org and SIOV_org for atarixl target
adapt to atarixl target
move ZPSAVE content into its own file (used by 2nd load chunk and crt0.s in atarixl target)
add more saved content
Handlers which manage the banking in and out of the ROM.
some more changes -- atarixl target basically works now
When turning the ROM off and on, also change the chargen pointer.
Take SHADOW_RAM size into account.
small cleanup
Merge remote-tracking branch 'upstream/master'
Merge remote-tracking branch 'upstream/master'
Merge remote-tracking branch 'upstream/master'
Use symbolic names for all CIO commands.
Merge remote-tracking branch 'upstream/master'

Conflicts:
	asminc/atari.inc