nixos-install: support -j, --cores and --option. Closes #6755

nixos/doc/manual/man-nixos-install.xml

@@ -25,6 +25,22 @@
       <arg choice='plain'><option>--root</option></arg>
       <replaceable>root</replaceable>
     </arg>
+    <arg>
+      <group choice='req'>
+        <arg choice='plain'><option>--max-jobs</option></arg>
+        <arg choice='plain'><option>-j</option></arg>
+      </group>
+      <replaceable>number</replaceable>
+    </arg>
+    <arg>
+      <option>--cores</option>
+      <replaceable>number</replaceable>
+    </arg>
+    <arg>
+      <option>--option</option>
+      <replaceable>name</replaceable>
+      <replaceable>value</replaceable>
+    </arg>
     <arg>
       <arg choice='plain'><option>--show-trace</option></arg>
     </arg>
@@ -96,6 +112,37 @@ it.</para>
     </listitem>
   </varlistentry>
 
+  <varlistentry><term><option>--max-jobs</option></term>
+    <term><option>-j</option></term>
+  
+    <listitem><para>Sets the maximum number of build jobs that Nix will
+    perform in parallel to the specified number. The default is <literal>1</literal>.
+    A higher value is useful on SMP systems or to exploit I/O latency.</para></listitem>
+  
+  </varlistentry>
+  
+  
+  <varlistentry><term><option>--cores</option></term>
+  
+    <listitem><para>Sets the value of the <envar>NIX_BUILD_CORES</envar>
+    environment variable in the invocation of builders.  Builders can
+    use this variable at their discretion to control the maximum amount
+    of parallelism.  For instance, in Nixpkgs, if the derivation
+    attribute <varname>enableParallelBuilding</varname> is set to
+    <literal>true</literal>, the builder passes the
+    <option>-j<replaceable>N</replaceable></option> flag to GNU Make.
+    The value <literal>0</literal> means that the builder should use all
+    available CPU cores in the system.</para></listitem>
+  
+  </varlistentry>
+
+  <varlistentry><term><option>--option</option> <replaceable>name</replaceable> <replaceable>value</replaceable></term>
+
+    <listitem><para>Set the Nix configuration option
+    <replaceable>name</replaceable> to <replaceable>value</replaceable>.</para></listitem>
+
+  </varlistentry>
+
   <varlistentry>
     <term><option>--show-trace</option></term>
     <listitem>

nixos/modules/installer/tools/nixos-install.sh

@@ -28,9 +28,14 @@ chrootCommand=(/run/current-system/sw/bin/bash)
 while [ "$#" -gt 0 ]; do
     i="$1"; shift 1
     case "$i" in
-        -I)
-            given_path="$1"; shift 1
-            extraBuildFlags+=("$i" "$given_path")
+        --max-jobs|-j|--cores|-I)
+            j="$1"; shift 1
+            extraBuildFlags+=("$i" "$j")
+            ;;
+        --option)
+            j="$1"; shift 1
+            k="$1"; shift 1
+            extraBuildFlags+=("$i" "$j" "$k")
             ;;
         --root)
             mountPoint="$1"; shift 1