uboot-config.bbclass: Raise an error for unknown configs

When passing the wrong value for UBOOT_CONFIG it ought to raise an
error otherwise it is quite difficult for user to notice it didn't
behave as expected.

Reported-by: Lauren Post <lauren.post@freescale.com>
(From OE-Core rev: 5110dc988e3b8a1ce8b2309e4ddf17abb3f1cb0c)

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

meta/classes/uboot-config.bbclass

@@ -51,4 +51,9 @@ python () {
             if len(items) > 1 and items[1]:
                 bb.debug(1, "Appending '%s' to IMAGE_FSTYPES." % items[1])
                 d.appendVar('IMAGE_FSTYPES', ' ' + items[1])
+
+            # Go out as we found a match!
+            break
+    else:
+        raise ValueError("UBOOT_CONFIG %s is not supported" % ubootconfig)
 }