kernel: enable magic sysrq

This can be useful via /proc/sysrq-trigger or by sending a break on
the serial.

modules/kernel/default.nix

@@ -131,6 +131,8 @@ in {
         # at boot time unless we disable trying to call it.
         # https://lkml.org/lkml/2013/8/5/175
         FW_LOADER_USER_HELPER = "n";
+
+        MAGIC_SYSRQ = "y";
       };
     };
   };