install: Pass "modprobe.blacklist=radeon".

* gnu/system/install.scm (installation-os)[kernel-arguments]: New field.

gnu/system/install.scm

@@ -427,6 +427,12 @@ (define installation-os
     (label (string-append "GNU Guix installation "
                           (package-version guix)))
 
+    ;; XXX: The AMD Radeon driver is reportedly broken, which makes kmscon
+    ;; non-functional:
+    ;; <https://lists.gnu.org/archive/html/guix-devel/2019-03/msg00441.html>.
+    ;; Thus, blacklist it.
+    (kernel-arguments '("modprobe.blacklist=radeon"))
+
     (file-systems
      ;; Note: the disk image build code overrides this root file system with
      ;; the appropriate one.