mk: add framework for defining inter-lib dependencies

Currently only the iscsi => scsi dependency is defined to
demonstrate how the framework behaves.

Eventually, lib/Makefile will be populated with more lines
of the form:

DEPDIRS-iscsi : scsi log conf event

This line will ensure that the iscsi subdirectory is built
after the specified dependency directories.

Then the lib/iscsi/Makefile also defines:

DEP_LIBNAMES = scsi log conf event

This line adds dependencies from the iscsi.so to the specified
libraries.

Ideally we could avoid this duplication by passing the DEPDIRS
value to the lower level Makefile, but that breaks down when we
start considering event/subsystems/* libraries.  So for now we'll
need to duplicate it.  Incidentally, DPDK also duplicates the
dependency information like this.

While here, remove use of $(MAKESUBDIRFLAGS) in spdk.subdirs.mk -
it was no longer used anywhere in the tree.

Signed-off-by: Jim Harris <james.r.harris@intel.com>
Change-Id: If63cbc10ffa18ac6361e4210bd6196ffe88bec43

Reviewed-on: https://review.gerrithub.io/429287


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Chandler-Test-Pool: SPDK Automated Test System <sys_sgsw@intel.com>
Reviewed-by: Changpeng Liu <changpeng.liu@intel.com>
Reviewed-by: Lance Hartmann <lance.hartmann@oracle.com>
Reviewed-by: Darek Stojaczyk <dariusz.stojaczyk@intel.com>
Reviewed-by: Seth Howell <seth.howell5141@gmail.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>