uboot-config.bbclass: Fix use without IMAGE_FSTYPES appending

Before checking if the image is a valid value, we need to check if the
list of items is big enough or we will raise an exception.

Reported-by: Lauren Post <lauren.post@freescale.com>
(From OE-Core rev: b833837cac377e7c1c3ff18531b152340b669329)

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/uboot-config.bbclass

@@ -47,7 +47,8 @@ python () {
             bb.debug(1, "Setting UBOOT_MACHINE to %s." % items[0])
             d.setVar('UBOOT_MACHINE', items[0])
 
-            if items[1]:
+            # IMAGE_FSTYPES appending
+            if len(items) > 1 and items[1]:
                 bb.debug(1, "Appending '%s' to IMAGE_FSTYPES." % items[1])
                 d.appendVar('IMAGE_FSTYPES', ' ' + items[1])
 }