app/testpmd: register driver specific commands

app/testpmd: register driver specific commands

Introduce a testpmd API so that drivers can register specific commands.

A driver can list some files to compile with testpmd, by setting them
in the testpmd_sources (driver local) meson variable.
drivers/meson.build then takes care of appending this to a global meson
variable, and adding the driver to testpmd dependency.

Note: testpmd.h is fixed to that it is self sufficient when being
included.

Signed-off-by: David Marchand <david.marchand@redhat.com>
Acked-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>