staging: Use relative path in sysroot-destdir for target recipes

staging: Use relative path in sysroot-destdir for target recipes

Original we used absolute path in sysroot-destdir for both native and
target recipes. This commit changes target recipes to use relative path
which is same as the image directory.

[sgw: merged with libtool sysroot work]
Signed-off-by: Dongxiao Xu <dongxiao.xu@intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>