meta-google: gbmc-bridge: Keep route sources across networkd reconfigures

networkd only keeps a route it configured when it reconfigures the
interface if the kernel route still matches the [Route] exactly,
including the protocol and the preferred source. gbmc-ra writes its
default routes without a source and gw-src adds the source afterwards
with ip(8), which also turns them into proto boot routes. So every
reconfigure of an uplink removes the route and adds it back without a
source, and the kernel sources front panel traffic from whatever address
it likes, e.g. the NCSI fd01:: address, until gw-src puts the source
back. The primary (metric 700) route is only installed with ip(8), so
networkd flushes it as a foreign route on every reconfigure. Updating
the source of the 1800 fallback also took the primary route down, as any
route through the same router removed it.

Save the source gw-src picks for each gbmc-ra route and write it into
the gateway drop-ins as PreferredSource=. gbmc-ra applies the saved
sources whenever it rewrites its drop-ins and installs its routes as
proto static with the source, and gw-src sets the source as proto static
too. The primary route goes into a gw-src drop-in on its interface. Only
the routes the primary route is made from remove it, and it is made from
the lowest metric route sourced from the primary IP. A route disabled
for lack of a source is only brought back if networkd still configures
it, rather than whenever the interface table still routes through its
gateway, which gbmc-ra keeps after dropping a router.

Tested: Unit tests for the drop-in helpers and a stubbed gw-src hook.
On head+tray systems with NCSI and front panel uplinks, forced networkd
to reconfigure each uplink and gbmcbr by adding a drop-in and reloading.
Before, a head lost its primary route for 12s on a front0 reconfigure.
After, no default route was deleted and all kept their source.

Change-Id: If4bc916efb0867705250d25db4470795018593d2
Signed-off-by: William A. Kennington III <wak@google.com>