tools: substitute bindir instead of BINDIR

tools: substitute bindir instead of BINDIR

... and same for sbindir and libdir.
Expand usage of exec_prefix so that it does not appear in substituted
variables in systemd files.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Akced-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
[ ijc -- ran autogen.sh as requested ]