Fixes bug 15464.
OECORE_NATIVE_SYSROOT is correctly set up and exported in the SDK's
environment file. But it's then unset in buildtools/environment-setup-*.
The value is restored in the SDK's environment file but is not exported
again.
(From OE-Core rev: 825c996b7995d3ad510933b1a88229831ca5ea29)
Signed-off-by: Gauthier HADERER <ghaderer@wyplay.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>