The new OFPT_ECHO_REQUEST and OFPT_ECHO_REPLY OpenFlow messages
have the same functionality and are more general (in that they
are not Netlink-specific).
We need object files to depend on the build directories, so that the
build directories get created before we try to output files into them.
But making them depend directly on the build directories means that
whenever a build directory changes (including any change to the set of
files inside it) all the source files get recompiled. So, instead, make
them depend on dummy files inside the directories, which will only get
modified if someone does it intentionally.
The new build system supports using a build tree separate from the
source tree, which makes it easier to build for multiple targets
using a single source tree. It also factors out the means for specifying
the modules to be built and the source files that comprise those
modules into simple files named Modules.mk. This is intended to
make it easier to add support for separate hardware table modules in
an upcoming commit.
Ben Pfaff
Ben Pfaff