target.cfg: remove "-work-area-virt 0"

The semantics of "-work-area-virt 0" (or phys) changed with
the patch to require specifying physical or virtrual work
area addresses.  Specifying zero was previously a NOP.  Now
it means that address zero is valid.

This patch addresses three related issues:

 - MMU-less processors should never specify work-area-virt;
   remove those specifications.  Such processors include
   ARM7TDMI, Cortex-M3, and ARM966.

 - MMU-equipped processors *can* specify work-area-virt...
   but zero won't be appropriate, except in mischievous
   contexts (which hide null pointer exceptions).

   Remove those specs from those processors too.  If any of
   those mappings is valid, someone will need to submit a
   patch adding it ... along with a comment saying what OS
   provides the mapping, and in which context.  Example,
   say "works with Linux 2.6.30+, in kernel mode".  (Note
   that ARM Linux doesn't map kernel memory to zero ...)

 - Clarify docs on that "-virt" and other work area stuff.

Seems to me work-area-virt is quite problematic; not every
operating system provides such static mappings; if they do,
they're not in every MMU context...

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>

tcl/target/imx27.cfg

@@ -40,7 +40,9 @@ jtag newtap $_CHIPNAME cpu  -irlen 4 -ircapture 0x1 -irmask 0xf -expected-id $_C
 # Create the GDB Target.
 set _TARGETNAME $_CHIPNAME.cpu
 target create $_TARGETNAME arm926ejs -endian $_ENDIAN -chain-position $_TARGETNAME -variant arm926ejs
-$_TARGETNAME configure -work-area-virt 0xffff4c00 -work-area-phys 0xffff4c00 -work-area-size  0x8000 -work-area-backup 1
+# REVISIT what operating environment sets up this virtual address mapping?
+$_TARGETNAME configure -work-area-virt 0xffff4c00 -work-area-phys 0xffff4c00 \
+	-work-area-size  0x8000 -work-area-backup 1
 # Internal to the chip, there is 45K of SRAM
 #