api: split api generated files

api: split api generated files

Split generation of API message enum and the typedefs into
separate files, so that the type file can be included from
elsewhere.

Generate a C file for VPP that contains the API registration,
this was previously done via X macros by the C pre-
processor.

This allows deleting lots of skeleton/copy paste code
for each feature.

plugins/flowprobe
plugins/map
examples/sample-plugin
vnet/ipip
used as Guinea pigs.

Generate a C Test file for VAT, that does the same for VAT plugins.
Also add support for a per-message CLI option, that is currently
limited to VAT help text. option vat_help = "<help text>";

Type: refactor
Change-Id: I245e3104bb76f7e1fb69a59ab20cc7c8dfcdd460
Signed-off-by: Ole Troan <ot@cisco.com>