package.bbclass: Use hard link for package split instead of copy

package.bbclass: Use hard link for package split instead of copy

When doing package split, we use hard link instead of copy, which can
save about 10% disk space when building poky-image-minimal.

If fail, it will fall back to the copyfile function.

[Updated by Richard to use os.link and avoid an exec() call per file]

Signed-off-by: Dongxiao Xu <dongxiao.xu@intel.com>