initramfs: eliminate custom $build variable

Previously the initramfs required overriders to use a custom
`$build` environment variable as the destination for files to be
packed into an initramfs image.  This commit eliminates it, so the
standard $NIX_BUILD_TOP can be used instead.

src/initramfs/default.nix

@@ -40,47 +40,44 @@ in stdenv.mkDerivation {
   passAsFile = [ "modulesList" ];
 
   buildPhase = ''
-    mkdir build
-    build=$(pwd)/build
-
     runHook preBuild
 
-    mkdir -p $build/lib/modules
+    mkdir -p $NIX_BUILD_TOP/lib/modules
     pushd ${kernel}/lib/modules/
-      cat $modulesListPath | ${cpio}/bin/cpio -p -d $build/lib/modules
+      cat $modulesListPath | ${cpio}/bin/cpio -p -d $NIX_BUILD_TOP/lib/modules
       popd
 
-    mkdir -p $build/usr
-    ln -s bin $build/sbin
-    ln -s ../bin $build/usr/bin
-    ln -s ../sbin $build/usr/sbin
+    mkdir -p $NIX_BUILD_TOP/usr
+    ln -s bin $NIX_BUILD_TOP/sbin
+    ln -s ../bin $NIX_BUILD_TOP/usr/bin
+    ln -s ../sbin $NIX_BUILD_TOP/usr/sbin
   '' + lib.optionalString withBusybox ''
-    cp -r ${busybox}/bin              $build/bin
-    chmod -R u+w $build/bin
+    cp -r ${busybox}/bin              $NIX_BUILD_TOP/bin
+    chmod -R u+w $NIX_BUILD_TOP/bin
   '' + lib.optionalString (bootScript != null) ''
-    cp    ${bootScript} $build/boot.sh
-    chmod +x $build/boot.sh
-    ln -s boot.sh $build/init
-    chmod +x $build/sbin/init
+    cp    ${bootScript} $NIX_BUILD_TOP/boot.sh
+    chmod +x $NIX_BUILD_TOP/boot.sh
+    ln -s boot.sh $NIX_BUILD_TOP/init
+    chmod +x $NIX_BUILD_TOP/sbin/init
   '' + lib.optionalString withSignify ''
-    cp    ${signify}/bin/signify      $build/bin/
+    cp    ${signify}/bin/signify      $NIX_BUILD_TOP/bin/
   '' + lib.optionalString withLvm ''
-    cp ${lib.getBin lvm2}/bin/lvm $build/bin/
-    cp ${lib.getBin lvm2}/bin/dmsetup $build/bin/
+    cp ${lib.getBin lvm2}/bin/lvm $NIX_BUILD_TOP/bin/
+    cp ${lib.getBin lvm2}/bin/dmsetup $NIX_BUILD_TOP/bin/
   '' + lib.optionalString withCryptsetup ''
-    cp ${cryptsetup}/bin/cryptsetup $build/bin/
+    cp ${cryptsetup}/bin/cryptsetup $NIX_BUILD_TOP/bin/
   '' + lib.optionalString withKexec ''
-    cp ${kexec-tools}/bin/kexec $build/bin/
+    cp ${kexec-tools}/bin/kexec $NIX_BUILD_TOP/bin/
   '' + lib.optionalString withNvramTool ''
-    cp ${nvramtool}/bin/nvramtool $build/bin/
+    cp ${nvramtool}/bin/nvramtool $NIX_BUILD_TOP/bin/
   '' + ''
     runHook postBuild
   '';
 
   installPhase = ''
     runHook preInstall
-    chmod -R u+w $build
-    pushd $build
+    chmod -R u+w $NIX_BUILD_TOP
+    pushd $NIX_BUILD_TOP
       ${findutils}/bin/find . | ${cpio}/bin/cpio --create -H newc -R +0:+0 > $out
       popd
     runHook postInstall

src/platform/kgpe/default.nix

@@ -19,9 +19,9 @@
 
     initramfs = prev.initramfs.overrideAttrs (a: {
       postBuild = (a.postBuild or "") + ''
-        mkdir -p $build/etc
-        cp ${final.image.src}/src/mainboard/asus/kgpe-d16/cmos.layout $build/etc/
-        cp ${final.image.src}/src/mainboard/asus/kgpe-d16/cmos.default $build/etc/
+        mkdir -p $NIX_BUILD_TOP/etc
+        cp ${final.image.src}/src/mainboard/asus/kgpe-d16/cmos.layout $NIX_BUILD_TOP/etc/
+        cp ${final.image.src}/src/mainboard/asus/kgpe-d16/cmos.default $NIX_BUILD_TOP/etc/
       '';
     });