Makefile: fix skipped dependencies

For some optional dependencies, both static and dynamic linking is tested.
But if the first one being tested fails, the dependency is added to the
NOTFOUND list and reported as being skipped while it might still be built
with another linkage.

Add optional dependencies to NOTFOUND only if both linkage are invalid.

Signed-off-by: Julien Thierry <julien.thierry@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>