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>

meta/classes/kernel.bbclass

@@ -185,7 +185,7 @@ kernel_do_install() {
 }
 
 sysroot_stage_all_append() {
-	sysroot_stage_dir ${D}/kernel ${SYSROOT_DESTDIR}${STAGING_KERNEL_DIR}
+	sysroot_stage_dir ${D}/kernel ${SYSROOT_DESTDIR}/kernel
 }