sstate.bbclass: make hard links for staging files

sstate.bbclass: make hard links for staging files

Make hard links for staging files instead of copy to save the disk space
(3G will be saved for a core-image-sato build), and it doesn't affect
much on the build time.

The following directories are affected:
1) The sysroot
2) The DEPLOY_DIR
3) The pkgdata

[YOCTO #4372]

(From OE-Core rev: 5853e0f482b22258c909268fe71673a29e31989b)

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>