smp: replace commands smp_on/smp_off with "smp [on|off]"

smp: replace commands smp_on/smp_off with "smp [on|off]"

Seams over-engineered having two separate commands to turn SMP
on/off. Plus it is missing the possibility to dump the current
status of SMP and would be weird adding an additional command
for it. Moreover, such commands are replicated in few targets so
it would make sense centralizing them.

- Deprecate the commands "smp_on" and "smp_off".
- Add a new command "smp" that accepts optional parameters
  "[on|off]" and prints the SMP status when run without
  parameters. This replaces the two commands above.
- Put the deprecated and the new command handlers in smp.c
- Update the documentation, except for mips_m4k, since it is not
  available yet.
- Promote the macro foreach_smp_target to global context and use
  it where possible.

Change-Id: Ia72841c1a3bd6edd4db4cc809046322f498617e6
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/4615
Tested-by: jenkins
Reviewed-by: Graham Sanderson <graham.sanderson@gmail.com>
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>