Add developer targets for each subdirectory we have something to build in

Add developer targets for each subdirectory we have something to build in

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: Stephen Henson <steve@openssl.org>