target: unify memory read/write commands

target: unify memory read/write commands

Current code provides two independent implementations for the
memory read/write commands:
a) jim_target_md()/jim_target_mw() for the target specific
   commands "<target> m[dw][bhw]"
b) handle_md_command()/handle_mw_command() for the current target
   commands "m[dw][bhwd]"
The case a) misses the handling of doubleword and calls functions
command_print_sameline() with first parameter NULL because defined
as jim handler.

Remove the code in jim_target_md()/jim_target_mw() and use the
same code in b) for the target specific memory read/write.
This approach also provides support for 64-bit data size, not
present in the implementation in a).
The only drawback is that the implementation of memory read in b)
prints the data also as human readable ASCII characters, feature
missing in the implementation in a). This could introduced later
if it's really needed.

Update the documentation to:
- add 64-bit support
- add phys parameter
- add the count parameter

Add attribute "unused" to target_buffer_get_u8(), now not used
anymore.

Change-Id: Ib11ff924c409ad87e77fe708d628b2cc82b74d6a
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5050
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>