Convert CONFIG_VGA_AS_SINGLE_DEVICE to Kconfig

This converts the following to Kconfig:
   CONFIG_VGA_AS_SINGLE_DEVICE

Once we migrate to driver model for video, we should be able to drop this
option.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/som-db5800-som-6867_defconfig b/configs/som-db5800-som-6867_defconfig
index 9fc169d..715311f 100644
--- a/configs/som-db5800-som-6867_defconfig
+++ b/configs/som-db5800-som-6867_defconfig
@@ -59,4 +59,5 @@
 CONFIG_VIDEO_VESA=y
 CONFIG_FRAMEBUFFER_SET_VESA_MODE=y
 CONFIG_FRAMEBUFFER_VESA_MODE_11A=y
+CONFIG_CFB_CONSOLE=y
 CONFIG_USE_PRIVATE_LIBGCC=y