syslinux: Fix syslinux_test in out-of-tree builds

syslinux: Fix syslinux_test in out-of-tree builds

syslinux_parse simplifies some filenames by removing things like ".."
segments, but the tests assumed that @abs_top_srcdir@ would be
untouched, which is not true in the case of out-of-tree builds where
@abs_top_srcdir@ may contain ".." segments.

Performing the substitution requires some awkwardness in Makefile.am due
to details of how config.status works.

Signed-off-by: Colin Watson <cjwatson@ubuntu.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>