helper/command: use one single handler for all the commands

helper/command: use one single handler for all the commands

Today openocd registers the commands to jim with three methods:
1) "native" commands (.jim_handler) at root level are registered
   directly as jim commands;
2) "simple" commands (.handler) at root level are registered
   through the handler script_command();
3) all other commands not at root level are registered through the
   handler command_unknown().

Apart from using different handler, other inconsistencies are
present:
a) command in 1) are not checked for their "mode", so are run with
   no check about current mode (COMMAND_CONFIG or COMMAND_EXEC);
b) target_call_timer_callbacks_now() is called only for "simple"
   commands and not for "native" commands;
c) target override is performed only for "simple" commands and not
   for "native" commands.

Drop script_command() and extend command_unknown() to uniformly
handle all the cases above, fixing all the inconsistencies already
mentioned.
The handler's name command_unknown() is probably not anymore
appropriate, but will be renamed in a separate change.

Note: today all the commands in a) have mode CONFIG_ANY, apart for
"mem2array" and "array2mem" that have mode COMMAND_EXEC. But the
latter commands are registered during target init, so do not exist
during COMMAND_CONFIG and no issue is present.

Change-Id: I67bd6e47eb2c575107251b9192c676c27d4aabae
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5665
Tested-by: jenkins
Reviewed-by: Oleksij Rempel <linux@rempel-privat.de>