populate_sdk_base.bbclass: allow $OECORE_NATIVE_SYSROOT in sdk_env_script

populate_sdk_base.bbclass: allow $OECORE_NATIVE_SYSROOT in sdk_env_script

Only grep for 'OECORE_NATIVE_SYSROOT=' otherwise things like

toolchain_create_sdk_env_script_append() {
    echo 'export MY_DIR_FOO=$OECORE_NATIVE_SYSROOT/my/dir/foo' >> $script
}

trigger the following error while executing the install script:

find: `$OECORE_NATIVE_SYSROOT/my/dir/foo': No such file or directory

Signed-off-by: Bernhard Guillon <Bernhard.Guillon@hale.at>
Signed-off-by: Saul Wold <sgw@linux.intel.com>