tbs2910: only enable vga output for stdout/stderr when hdmi detected

Only enable graphical output for stdout/stderr (and a usb keyboard for stdin)
when a hdmi device is detected.
Serial console is always enabled for stdin/stdout/stderr.

Signed-off-by: Soeren Moch <smoch@web.de>

include/configs/tbs2910.h

@@ -183,7 +183,13 @@
 #ifdef CONFIG_USB_KEYBOARD
 #define CONFIG_SYS_USB_EVENT_POLL_VIA_INT_QUEUE
 #define CONFIG_SYS_STDIO_DEREGISTER
-#define CONFIG_PREBOOT "if hdmidet; then usb start; fi"
+#define CONFIG_PREBOOT \
+	"if hdmidet; then " \
+		"usb start; " \
+		"run set_con_usb_hdmi; " \
+	"else " \
+		"run set_con_serial; " \
+	"fi;"
 #endif /* CONFIG_USB_KEYBOARD */
 #endif /* CONFIG_CMD_USB      */
 
@@ -242,9 +248,12 @@
 			"bootm 0x10800000 0x10d00000\0" \
 	"console=ttymxc0\0" \
 	"fan=gpio set 92\0" \
-	"stdin=serial,usbkbd\0" \
-	"stdout=serial,vga\0" \
-	"stderr=serial,vga\0"
+	"set_con_serial=setenv stdin serial; " \
+			"setenv stdout serial; " \
+			"setenv stderr serial;\0" \
+	"set_con_usb_hdmi=setenv stdin serial,usbkbd; " \
+			"setenv stdout serial,vga; " \
+			"setenv stderr serial,vga;\0"
 
 #define CONFIG_BOOTCOMMAND \
 	"mmc rescan; " \