Control plane agent for Nucleo boards

[lzrd]

Improve Nucleo support for Hubris and hardware testing

The Nucleo board is a useful off-the-shelf board for prototyping and for
testing "pure Hubris" code paths. Configuring the control plane agent
into the image makes the board more useful.

The Nucleo board can also be a convenient platform for developing I2C
device drivers.

While there is enforcement in xtask to ensure that the vlan feature is
enabled for production boards, the feature results in compilation errors
if those guards are commented out and the vlan feature is removed from
the app.toml. That is fixed so that the Nucleo boards can opt out of
the vlan feature.

All of the changes in the machine code output are explained by line
number changes (ringbuf, etc.), the caboose GITC value, a different
Hubris ImageId, and one layout change due to a line number constant value
being a different size: in control_plane_agent in handle_mgs_request
a cmp.w r0, #728 became move r1,#729; cmp r0, r1

While people have prototyped a Nucleo + NXP LPC55S69 xPresso board to
simulate SP + RoT, there is no intention to officially support that
configuration.

Configuration changes to Nucleo boards:

• add control_plane_agent (CPA)
• add "Caboose" for Hubris image identification
• omitting VLAN tags on Nucleo boards
• update_server to manage firmware updates
• sprot is assigned the appropriate pins for connection to a non-existent RoT

control_plane_agent references i2c_driver unconditionally via
task_slot!(I2C, i2c_driver), so add it to CPA's task-slots in
app-h743.toml (app-h753.toml already had it).