platform/mulle: Cleanup in tools/openocd.

Renamed mulle.cfg -> mulle-programmer-0.60.conf
Renamed mulle-programmer-0.70.cfg -> mulle-programmer-0.70.conf
Symlinked new file to mulle.cfg for backwards compatibility.

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
platform/mulle: OpenOCD config from environment.

Specify PROGRAMMER_VERSION or PROGRAMMER_SERIAL to choose a specific
programmer board.

PROGRAMMER_SERIAL should be set to the number on the sticker on the
programmer board.
PROGRAMMER_VERSION can be used instead if the serial number filtering
is not desired, e.g. when swapping USB cables between two separate
programmers of the same revision.
The version of the programmer is listed in the USB device name, e.g.
from dmesg:

    usb 2-2.2: Product: Mulle Programmer v0.60
    usb 2-2.2: Manufacturer: Eistec AB
    usb 2-2.2: SerialNumber: 100

Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>