at91bootstrap: do not default to $(BOARD_NAME)

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

boot/at91bootstrap/Config.in

@@ -6,7 +6,7 @@ if	BR2_TARGET_AT91BOOTSTRAP
 
 config BR2_TARGET_AT91BOOTSTRAP_BOARD
 	string "Bootstrap board"
-	default "$(BOARD_NAME)"
+	default ""
 	help
 	  This is used to do a make <board>_config