gitmodules: Fix 3rdparty updates

Commit 039223a: gitmodules: Ignore 3rdparty in "diff family"
changed the behavior of our 3rdparty repository and disallowed
updates to the checked out hash. Instead of "ignore=all" we
want "ignore=dirty" to ignore local changes but allow changing
to the HEAD of the 3rdparty repo.

Change-Id: I66c35ad4fcfb0efb0ba611f67648a096a6de1479
Signed-off-by: Stefan Reinauer <[email protected]>
Reviewed-on: http://review.coreboot.org/3566
Reviewed-by: Ronald G. Minnich <[email protected]>
Tested-by: Stefan Reinauer <[email protected]>

.gitmodules

@@ -2,4 +2,4 @@
 	path = 3rdparty
 	url = ../blobs.git
 	update = none
-	ignore = all
+	ignore = dirty