Add Dell E7240 Profile

(Current XPS 15 works well)

dell/e7240/README.md

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+On some kernel versions user ashgillman has experiences suspend issues
+(see https://bugzilla.redhat.com/show_bug.cgi?id=1597481).
+
+Try:
+
+```nix
+boot.kernelPackages = pkgs.linuxPackages_4_14;
+```

dell/e7240/default.nix

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+{ lib, pkgs, ... }:
+
+{
+  imports = [
+    ../../common/cpu/intel
+    ../../common/pc/laptop
+  ];
+}