boards: adjust openocd runner arg syntax

The current method of setting the openocd runner arguments works for
direct invocation from the command line, but the values that are
written to the CMake cache are not properly separated and are
difficult to parse, due to the fact that in several cases, option
values contain spaces.

Adjust these runner arguments so that the options and their values are
separate list items. This makes it possible to parse them accurately
from the CMake cache.

No functional changes.

Signed-off-by: Marti Bolivar <marti@opensourcefoundries.com>