make dde_linux repository an optional one

Since dde_linux now contains the port of the Linux IP stack available for all
Genode base-* platforms move the repository out of drivers_arm and drivers_x86
build.conf to the optional build.conf (available to all platforms).

tool/builddir/etc/build.conf.drivers_arm

@@ -1,5 +1 @@
 
-#
-# Drivers ported from the Linux kernel (USB)
-#
-#REPOSITORIES += $(GENODE_DIR)/dde_linux

tool/builddir/etc/build.conf.drivers_x86

@@ -1,11 +1,4 @@
 
-#
-# Drivers ported from the Linux kernel (USB)
-#
-# Not supported on the ARM architecture.
-#
-#REPOSITORIES += $(GENODE_DIR)/dde_linux
-
 #
 # Additional drivers ported from the Linux kernel (audio, Intel GEM)
 #

tool/builddir/etc/build.conf.optional

@@ -26,6 +26,11 @@
 #
 #REPOSITORIES += $(GENODE_DIR)/ports
 
+#
+# Ports of the Linux TCP/IP stack and Linux drivers (USB) from the Linux kernel
+#
+#REPOSITORIES += $(GENODE_DIR)/dde_linux
+
 #
 # High-level Genode-specific services and applications
 #