nativesdk.bbclass: set consistent staging dirs regardless of multilib

For now, the RECIPE_SYSROOT of nativesdk recipes is ${WORKDIR}/recipe-sysroot
if multilib is disabled and ${WORKDIR}/nativesdk-recipe-sysroot if multilib
is enabled. And it's causing chaos. Problems I've met include:
1) 'File Exists' error when doing extend_recipe_sysroot
2) Rebuilding failure about cmake based nativesdk recipes if toggling multilib

In nativesdk.bbclass, We've set MULTILIBS to be "", and we've changed MLPREFIX
to be 'nativesdk-', I think we should also set consistent RECIPE_SYSROOT to be
${WORKDIR}/recipe-sysroot.

Below is an example showing why previous settings will cause do_prepare_recipe_sysroot
failure.

e.g.
A -> C
B -> C
A's RECIPE_SYSROOT is .../recipe-sysroot and B's RECIPE_SYSROOT is
.../nativesdk-recipe-sysroot.
As extend_recipe_sysroot function uses shared manifest, i.e., the same
manifest of C for both A and B, then there must be one of them having
the wrong manifest. And the wrong manifest results in RECIPE_SYSROOT
not cleaned up before installing new components, thus the following error.

  Exception: FileExistsError: [Errno 17] File exists: xxx -> xxx

This happens when toggling multilib and also between nativesdk recipes and
crosssdk, cross-canadian recipes. The latter situation also explains
why choosing ${WORKDIR}/recipe-sysroot instead of ${WORKDIR}/nativesdk-recipe-sysroot.
If we use 'nativesdk-recipe-sysroot', we still need to modify the extend_recipe_sysroot
function to treat crosssdk and cross-canadian as special cases. Using
'recipe-sysroot' does not have this problem.

(From OE-Core rev: 665934a506cc560bfbc469f5ed095e7d54e353a5)

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/nativesdk.bbclass

@@ -12,6 +12,11 @@ MACHINEOVERRIDES = ""
 
 MULTILIBS = ""
 
+# we need consistent staging dir whether or not multilib is enabled
+STAGING_DIR_HOST = "${WORKDIR}/recipe-sysroot"
+STAGING_DIR_TARGET = "${WORKDIR}/recipe-sysroot"
+RECIPE_SYSROOT = "${WORKDIR}/recipe-sysroot"
+
 #
 # Update PACKAGE_ARCH and PACKAGE_ARCHS
 #