meta-flow: Autogenerate mf_field data structures.

This is a first step toward improving the abstraction of OXM and NXM in the
tree.  As an immediate improvement, this commit removes all of the
definitions of the OXM and NXM constants from the top-level header files,
because they are no longer used anywhere.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>