makefiles/boot/riotboot.mk: exclude if RIOTBOOT_BUILD is set

- riotboot targets should not be needed for riotboot application
  so dont include it.
- also fixes #12003 by not setting FLASHFILE = $(RIOTBOOT_EXTENDED_BIN)
  when compiling riotboot application

makefiles/boot/riotboot.mk

@@ -1,4 +1,5 @@
 ifneq (,$(filter riotboot,$(FEATURES_USED)))
+ifneq (1,$(RIOTBOOT_BUILD))
 
 .PHONY: riotboot/flash riotboot/flash-bootloader riotboot/flash-slot0 riotboot/flash-slot1 riotboot/bootloader/%
 
@@ -154,4 +155,5 @@ riotboot:
 	$(Q)echo "error: riotboot feature not selected! (try FEATURES_REQUIRED += riotboot)"
 	$(Q)false
 
+endif # (1,$(RIOTBOOT_BUILD))
 endif # (,$(filter riotboot,$(FEATURES_USED)))