platform/kevin: eliminate path-reference to nixpkgs

Now that 6deb86f6138265a715dd005c310ad33a2e2865ff has merged
upstream in nixpkgs we can directly reference arm-trusted-firmware
as part of the nixpkgs package set.  This allows to eliminate a
<nixpkgs>-style path reference.

src/platform/kevin/default.nix

@@ -67,7 +67,7 @@ in {
 
     coreboot =
       let atf = final.nixpkgsOnBuildForHost.callPackage (import ./atf) {
-            buildArmTrustedFirmware = (final.nixpkgsOnBuildForHost.callPackage (import <nixpkgs/pkgs/misc/arm-trusted-firmware>) {
+            buildArmTrustedFirmware = (final.nixpkgsOnBuildForHost.arm-trusted-firmware.override {
               unfreeIncludeHDCPBlob = false;
               openssl = null;    # not sure why they would link in openssl...
             }).buildArmTrustedFirmware;