Previous build scheme allowed building just the stuff in one
subdirectory, like this:

    make -C crypto/aes

Because the unified only has a top-level Makefile, this is not
possible with it.  This change adds a replacement where each directory
we have something to build in becomes a target in its own right,
allowing building something like this:

    make crypto/aes

The exception is the directory test, because we already have such a
target.
Reviewed-by: NStephen Henson <steve@openssl.org>