openocd: clean up obsolete openocd install targets

The old 'openocd_install' target was still installing the obsolete version of the openocd tools that no longer work with our boards.  The old version has been removed.

The 'openocd_git_install' target is now renamed as 'openocd_install' which now matches the help text.

I've also pushed an update to the Linux development environment wiki page to document how to build openocd.

This fixes #935 reported by @timgoh0 a while ago.