support for scripts in $HOME/.openocd

Add $HOME/.openocd as the first default script search directory, allowing
the user to override the standard scripts.

Update the user guide with information on where OpenOCD expects to find
configuration files and scripts. Also fixed some minor formatting issues.

Add entry to NEWS as well.

Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Signed-off-by: Øyvind Harboe <oyvind.harboe@zylin.com>