kgpe: allow to configure recovery nvram values

On amd64 platforms, booting ownerboot with the recovery jumper
installed will wipe the battery-backed nvram (aka "cmos" aka "rtc
nvram") and overwrite it with known-safe values taken from the
coreboot source code (`src/mainboard/asus/kgpe-d16/cmos.default`).

You should always do this when flashing a motherboard with ownerboot
for the first time.

This commit allows the user to customize the set of known-safe
values which are written when the recovery jumper is installed.  To
do so, copy `src/mainboard/asus/kgpe-d16/cmos.default` out of
coreboot, edit to suit your tastes, and then override
`cmos-defaults` with the path to your customized `cmos.default`
file.

src/platform/kgpe/default.nix

@@ -33,6 +33,16 @@
     # insecure configurations? (63xx without microcode)
     microcode-blob = null;
 
+    # This contains the default values written into the
+    # battery-backed nvram when you boot with the recovery jumper
+    # installed (which is something you should do the first time you
+    # boot with ownerboot/coreboot).  If you want to customize these
+    # values, copy `src/mainboard/asus/kgpe-d16/cmos.default` out of
+    # coreboot and edit to suit your tastes.
+    #
+    # TODO: use a NixOS-style structuredConfig for this.
+    cmos-default = null;
+
     coreboot = final.lib.makeOverridable (prev.coreboot.override {
       iasl = final.iasl_20180531;
       payload = "${final.kernel}/bzImage";
@@ -52,6 +62,8 @@
           3rdparty/blobs/cpu/amd/family_15h/microcode_amd_fam15h.bin
         mkdir -p 3rdparty/blobs/cpu/amd/family_10h-family_14h/
         touch 3rdparty/blobs/cpu/amd/family_10h-family_14h/microcode_amd.bin
+      '' + final.lib.optionalString (final.cmos-default != null) ''
+        cp ${final.cmos-default} src/mainboard/asus/kgpe-d16/cmos.default
       '';
       postInstall = (a.postInstall or "") + ''
         cp src/mainboard/asus/kgpe-d16/cmos.layout $out/