neon: explicit path to xml2-config

Now that $(STAGING_DIR)/usr/bin is not in the PATH, we need to
explicit the path to such tools.

This might also fix bug 1393 since by expliciting the path to
xml2-config, we'll force neon ./configure to choose our xml2-config
and not the one of the host.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>