makefiles/boot/riotboot: pass BUILD_DIR to recursive Make call

Author: mguetschow

makefiles/boot/riotboot.mk

@@ -86,6 +86,7 @@ riotboot/flash-bootloader: riotboot/bootloader/flash
 riotboot/bootloader/%: $$(if $$(filter riotboot/bootloader/clean,$$@),,$$(BUILDDEPS) pkg-prepare)
 	$(Q)/usr/bin/env -i \
 		QUIET=$(QUIET) PATH="$(PATH)" USER="$(USER)"\
+		BUILD_DIR="$(BUILD_DIR)"\
 		INCLUDES="$(INCLUDES)"\
 		EXTERNAL_BOARD_DIRS="$(EXTERNAL_BOARD_DIRS)" BOARD=$(BOARD) \
 		OPENOCD_DEBUG_ADAPTER=$(OPENOCD_DEBUG_ADAPTER) \