Fix --enable-linux-builtin

Adding VPATH support, commit 37d7cd9, required that a src
and obj line be added to the top of the Makefiles. They
must be removed from the Makefiles when builtin.

The code which adds the spl/ directory to the top level
Makefile was failing due to the addition of the certs/ path.
The search pattern has been adjusted to be more tolerant.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #481
Issue #498