target: make target re-configuration possible again

target: make target re-configuration possible again

Before commit 877cec20dca6e78f9f029f0f173879cda101a6c2
("command: check command mode for native jim commands") all the jim commands
were erroneously treated as they had mode COMMAND_ANY.

The command '$_TARGET configure -xxx' was therefore applicable on running
OpenOCD to change the target configuration. It is handy e.g. for changing
an event handler or changes of the work area.

Change 'configure' command .mode to COMMAND_ANY to make it possible again.

The only parameter which cannot be re-configured after init is -gdb-port.
Test the command mode and refuse setting of gdb port after init.

Change-Id: I88493ac10a46647dc52a88fbc9f8ce6b5ba3bcd0
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/5214
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Tested-by: jenkins
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>