support setting network device names

this means that net devices in devices/foo/default.nix can be
specified by their sysfs paths (instead of by "eth0" and "eth1" that
may change from one kernel version to the next) and given mnenomic
names that are helpful for the hardware. Like "wan" and "lan[1..4]"

modules/network/link.nix

@@ -4,13 +4,27 @@
 , serviceFns
 , lib
 }:
-{ifname, mtu} :
+{
+  ifname
+, devpath ? null
+, mtu} :
+# if devpath is supplied, we rename the interface at that
+# path to have the specified name.
 let
   inherit (liminix.services) longrun oneshot;
   inherit (lib) concatStringsSep;
   name = "${ifname}.link";
-  up = liminix.networking.ifup name ifname;
+  rename = if devpath != null
+           then ''
+             oldname=$(cd /sys${devpath} && cd net/ && echo *)
+             ip link set ''${oldname} name ${ifname}
+           ''
+           else "";
 in oneshot {
-  inherit name up;
+  inherit name;
+  up = ''
+    ${rename}
+    ${liminix.networking.ifup name ifname}
+  '';
   down = "ip link set down dev ${ifname}";
 }