Add ability to say a custom partition doesn't want to participate in fastboot flashall

Bug: 399738266
Test: treehugger
Flag: EXEMPT this is a flag!
Change-Id: I59406e9877f8399b90a7dced53eec9decd2a8567
diff --git a/core/Makefile b/core/Makefile
index 0d6b175..e6d687d 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -5885,7 +5885,10 @@
 endif # BOARD_AVB_ENABLE
 ifneq (,$(strip $(BOARD_CUSTOMIMAGES_PARTITION_LIST)))
 	$(hide) $(foreach partition,$(BOARD_CUSTOMIMAGES_PARTITION_LIST), \
-	  echo "flash $(partition)" >> $@;)
+		$(if $(BOARD_$(call to-upper,$(partition))_IMAGE_NO_FLASHALL),, \
+	      echo "flash $(partition)" >> $@; \
+		) \
+	)
 endif
 	$(hide) echo "reboot fastboot" >> $@
 	$(hide) echo "update-super" >> $@