keep build files for each board

ready for signing

RemoteIDModule/Makefile

@@ -40,11 +40,11 @@ headers:
 ArduRemoteID-%.bin: *.cpp *.ino *.h
 	@echo "Building $* on $(CHIP)"
 	@BUILD_FLAGS="-DBOARD_$*"
-	@rm -rf build
+	@rm -rf build build-$*
 	@$(ARDUINO_CLI) compile -b esp32:esp32:$(CHIP) --export-binaries --build-property build.extra_flags=-DBOARD_$* .
 	@echo "Merging $*"
 	@python3 $(ESPTOOL) --chip $(CHIP) merge_bin -o ArduRemoteID-$*.bin --flash_size 4MB 0xe000 $(ESP32_TOOLS)/partitions/boot_app0.bin 0x0 build/esp32.esp32.$(CHIP)/RemoteIDModule.ino.bootloader.bin 0x10000 build/esp32.esp32.$(CHIP)/RemoteIDModule.ino.bin 0x8000 build/esp32.esp32.$(CHIP)/RemoteIDModule.ino.partitions.bin
-	@rm -rf build
+	@mv build build-$*
 
 boards:
 	@echo "Listing boards"