dummycon: only build module if there are users

Dummycon is used as a fallback conswitchp for vgacon and fbcon
in the VT code, and there are no references to it if all three
are disabled, so just leave it out of the kernel image for
configurations without those.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Helge Deller <deller@gmx.de>

Author: arndb

drivers/video/console/Kconfig

@@ -47,8 +47,7 @@ config SGI_NEWPORT_CONSOLE
 	  card of your Indy.  Most people say Y here.
 
 config DUMMY_CONSOLE
-	bool
-	default y
+	def_bool VT || VGA_CONSOLE || FRAMEBUFFER_CONSOLE
 
 config DUMMY_CONSOLE_COLUMNS
 	int "Initial number of console screen columns"