fix the symlink() command to create directories if needed

Full OTAs currently fail if the build contains a directory containing
only symlinks, because nothing creates that directory.  Change the
symlink() command to create any ancestor directories that don't exist.
They're created as owner root perms 0700 because we assume that in
practice subsequent set_perm_recursive() calls will fix up their
ownership and permissions.

Change-Id: I4681cbc85863d9778e36b924f0532b2b3ef14310