Upstream AM_PATH_XML2 uses xml2-config which we disable, so port this macro to
use pkg-config.
(From OE-Core rev: 3ea77e69a839572a948ff6f1e51d3ca789ad8eed)
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>