diff --git a/makefiles/boards.inc.mk b/makefiles/boards.inc.mk
index 15b67f7803..7d7374d1af 100644
--- a/makefiles/boards.inc.mk
+++ b/makefiles/boards.inc.mk
@@ -12,8 +12,8 @@ ifneq ($(RIOTBOARD),$(BOARDSDIR))
   ALLBOARDS_RIOTBOARD ?= $(call _get_boards_in_directory,$(RIOTBOARD))
 endif
 
-# Get all boards
-ALLBOARDS ?= $(sort $(call _get_boards_in_directory,$(BOARDSDIR)) $(ALLBOARDS_RIOTBOARD))
+# Use `:=` so that it is evaluated before BOARDSDIR gets eventually changed
+ALLBOARDS := $(sort $(call _get_boards_in_directory,$(BOARDSDIR)) $(ALLBOARDS_RIOTBOARD))
 
 # Set the default value from `BOARDS`
 BOARDS ?= $(ALLBOARDS)