configure: add alternate binary for genisoimage

Not all distros ship genisoimage which is a Debian fork from the
original cdrtools. As the options are pretty much the same support it
as a fallback binary.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Robert Foley <robert.foley@linaro.org>
Message-Id: <20200519132259.405-2-robert.foley@linaro.org>

configure

@@ -941,7 +941,7 @@ done
 
 # Check for ancillary tools used in testing
 genisoimage=
-for binary in genisoimage
+for binary in genisoimage mkisofs
 do
     if has $binary
     then