Export UARTs to Ariel

[chrysn]

(Built on #67, please ignore the first two commits)

This builds up UARTs to be useful with Ariel OS in a way that avoids the need for pins.rs in applications (which I think is part of the purpose of board descriptions: to enable generic applications to run on any board, no matter whether the application or the board description is out-of-tree).

Concrete changes:

• I'm making serde strict. If this is controversial, I'll move it out into another commit, but I think it matters, for Ariel has already started racking up various "alternative syntaxes" for active-high/low. (Those are not cleaned up here yet, just avoiding the issue gets worse)
• "host_facing" is defined as a new property. This is an important property, because boards may have all kinds of UARTs (to on-board devices, to extension ports, to the debugger or to USB), and the latter two are the ones an application that offers, say, a shell, or debug output, or (in my case) a serial line network uplink wants to select.
  • In an extra commit, that is turned into a has_ property, so laze can filter on such a port's presence to decide if an application makese sense to build there.
• Allow annotating which UART devices go with which UART-as-defined-so-far (i.e., pair of TX/RX pins). See comment there -- at some point, this could be automated.
• Finally, UARTs get exposed to Ariel. Conveniently, this change does not need to lockstep with Ariel, because as long as no devices declare their MCU periph, nothing gets gen'd