mk/lib: Build subdirs before lib dir

The subdirectories of libraries are currently listed as dependencies of
the library. If you look at the actual symbols they contain, the
opposite is true. For all of the bdevs, event subsystems and other
library directories, the subdirectories actually rely on symbols
contained in the parent directory.
If we want to create the proper inter-library dependencies in SPDK at
compile/link time, then we need to respect the order of these symbol
dependencies when executing make (e.g. make libspdk_event.so before
libspdk_event_bdev.so). Otherwise we create a loop of dependencies in
the make system.

Change-Id: Idcbb050b2709f575d0cec46b481824d6313612b4
Signed-off-by: Seth Howell <seth.howell@intel.com>
Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/465191


Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>