JLink target power

[Rolf | Onethinx]

Hi,

There is a problem with setting JLink power on. JLink has a possibility to feed 
the target hardware from the J-Link debugger.

There is a command already in OpenOCD: `jlink config targetpower on`, however 
the current implementation does not work. The target power has to be ON before 
the `init` command because the init command needs to connect to the chip. 
Currently this is not allowed: `The ‘link config targetpower’ command must be 
used after ‘init’`. This does not make sense as init will fail when there’s no 
power applied to the target chip.

I did some changes to jlink.c in order to make it work, however I’m not 
familiar with the OpenOCD development and I’m unsure about the correct coding 
style for OpenOCD.

I did these modifications to /jtag/drivers/jlink.c for version 0.12.0-rc2+dev:

Changed the command mode of  `jlink config targetpower` from COMMAND_EXEC to 
COMMAND_CONFIG (line 1822)

Inserted the following code at line 754 (just after the check of the J-Link 
hardware version) to check if the  `jlink config targetpower on` has been 
issued:
```
        if (tmp_config.target_power)
        {
                ret = jaylink_set_target_power(devh, true);

                if (ret != JAYLINK_OK) {
                        LOG_ERROR("jaylink_set_target_power() failed: %s",
                                jaylink_strerror(ret));
                        jaylink_close(devh);
                        jaylink_exit(jayctx);
                        return ERROR_JTAG_INIT_FAILED;
                }
                jtag_sleep(50000);                      // Wait for 50mS for 
power to settle
        }
```

This works perfect and I hope this modification (or similar) can be considered 
for next update.

Thanks!
Rolf