linux/common-config: enable AX25

This will allow dropping the linuxPackages_ham variant.  Fedora sets
these in their default kernel, so I don't anticipate any problem with
us doing the same.

Tested building linux_5_4, linux, and linux_latest on x86_64.

(cherry picked from commit d735743b39)

nixos/tests/ax25.nix

@@ -8,8 +8,6 @@ import ./make-test-python.nix (
     socatPort = 1234;
 
     createAX25Node = nodeId: {
-
-      boot.kernelPackages = pkgs.linuxPackages_ham;
       boot.kernelModules = [ "ax25" ];
 
       networking.firewall.allowedTCPPorts = [ socatPort ];

pkgs/os-specific/linux/kernel/common-config.nix

@@ -395,6 +395,10 @@ let
       # Enable debugfs for wireless drivers
       CFG80211_DEBUGFS = yes;
       MAC80211_DEBUGFS = yes;
+
+      # HAM radio
+      HAMRADIO = yes;
+      AX25 = module;
     }
     // lib.optionalAttrs (stdenv.hostPlatform.system == "aarch64-linux") {
       # Not enabled by default, hides modules behind it