populate_sdk_ext: Ensure buildtools doesn't corrupt OECORE_NATIVE_SYSROOT

buildtools is built as a nativesdk which needs to use OECORE_NATIVE_SYSROOT
for its own purposes and can reset it. Save and restore the value
within the eSDK so the two don't clash.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>