blktap2 is optional, so there should be no pkg-config file requiring
xenblktapctl if it isn't enabled for the build.
Add a filter mechanism to tools/Rules.mk to filter out optional
libraries.
Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Release-acked-by: Julien Grall <julien.grall@arm.com>