initrd: add mmc_block to default available modules

mmc_block and sdhci_acpi are both necessary for a Bay Trail Chromebook with an
internal eMMC drive. The sdhci_acpi module is detectable but I can not figure
out a way to check whether the mmc_block module is needed by just looking at
/sys/

nixos/modules/installer/cd-dvd/system-tarball-sheevaplug.nix

@@ -86,8 +86,7 @@ in
   system.boot.loader.kernelFile = "uImage";
 
   boot.initrd.availableKernelModules =
-    [ "mvsdio" "mmc_block" "reiserfs" "ext3" "ums-cypress" "rtc_mv"
-      "ext4" ];
+    [ "mvsdio" "reiserfs" "ext3" "ums-cypress" "rtc_mv" "ext4" ];
 
   boot.postBootCommands =
     ''

nixos/modules/installer/cd-dvd/system-tarball.nix

@@ -43,7 +43,7 @@ in
     # so that we don't need to know its device.
     fileSystems = [ ];
 
-    # boot.initrd.availableKernelModules = [ "mvsdio" "mmc_block" "reiserfs" "ext3" "ext4" ];
+    # boot.initrd.availableKernelModules = [ "mvsdio" "reiserfs" "ext3" "ext4" ];
 
     # boot.initrd.kernelModules = [ "rtc_mv" ];
 

nixos/modules/system/boot/kernel.nix

@@ -184,6 +184,9 @@ in
         "ide_disk"
         "ide_generic"
 
+        # SD cards and internal eMMC drives.
+        "mmc_block"
+
         # Support USB keyboards, in case the boot fails and we only have
         # a USB keyboard.
         "uhci_hcd"