added hardware.ram.startAddress config

it's not 0 on arm32, so this will be useful for qemu

modules/arch/mips.nix

@@ -10,6 +10,7 @@
       OF = "y";
       USE_OF = "y";
     };
+    hardware.ram.startAddress = 0;
     boot.commandLine = [
       "console=ttyS0,115200" # true of all mips we've yet encountered
     ];

modules/hardware.nix

@@ -29,6 +29,12 @@ in {
         description = "\"Default\" output: what gets built for this device when outputs.default is requested. Typically this is \"flashimage\" or \"vmroot\"";
         type = types.nonEmptyStr;
       };
+      ram = {
+        startAddress = mkOption {
+          type = types.int;
+        };
+      };
+
       flash = {
         # start address and size of whichever partition (often
         # called "firmware") we're going to overwrite with our