Support an Arch-style Arduino installation

Arch puts the Arduino tools into /usr/bin, hardware bits under
/usr/share/arduino, and so on. To make ends meet, and support both the
traditional install, and Arch, introduce a bit of magic:

- If `ARDUINO_TOOLS_PATH` is empty, then we should not add the `-tools`
  param that refers to it. On arch, this is not needed, and there is no
  reasonable alternative, that would also make sense. So in this case,
  it should not be added at all. Setting the variable to an empty string
  accomplishes that goal nicely.

- If an `AVR_GCC_PREFIX` variable is set, use that as the value for
  `runtime.tools.avr-gcc.path`, otherwise `arduino-builder` will try to
  find the avr-* tools somewhere under the Arduino prefix, which on
  Arch, is not the case.

With this change, along with keyboardio/KeyboardioHID#3, and adding an
`archlinux-arduino`=>`arduino` symlink somewhere, it becomes possible to
complie KeyboardioFirmware on Arch, using the packaged Arduino, with the
following commandline:

> make ARDUINO_BUILDER_PATH=/usr/bin/arduino-builder \
>      ARDUINO_PATH=/usr/share/arduino \
>      ARDUINO_LOCAL_LIB_PATH=../arduino-local \
>      AVR_GCC_PREFIX=/usr \
>      ARDUINO_TOOLS_PATH= \
>      AVR_SIZE_PATH=avr-size

Not the nicest, by far, but possible.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>

Makefile

@@ -55,6 +55,15 @@ SKETCH=KeyboardioFirmware.ino
 BOOTLOADER_PATH = $(ARDUINO_LOCAL_LIB_PATH)/hardware/keyboardio/avr/bootloaders/caterina/Caterina.hex
 VERBOSE= #-verbose
 
+ARDUINO_TOOLS_PARAM = -tools $(ARDUINO_TOOLS_PATH)
+ifeq ($(ARDUINO_TOOLS_PATH),)
+	ARDUINO_TOOLS_PARAM =
+endif
+
+ifdef AVR_GCC_PREFIX
+	ARDUINO_AVR_GCC_PREFIX_PREF = -prefs "runtime.tools.avr-gcc.path=$(AVR_GCC_PREFIX)"
+endif
+
 #
 #
 # Build
@@ -96,7 +105,7 @@ compile: dirs
 	$(ARDUINO_BUILDER_PATH) \
 		-hardware $(ARDUINO_PATH)/hardware \
 		-hardware $(ARDUINO_LOCAL_LIB_PATH)/hardware \
-		-tools $(ARDUINO_TOOLS_PATH) \
+		$(ARDUINO_TOOLS_PARAM) \
 		-tools $(ARDUINO_PATH)/tools-builder  \
 		-fqbn $(FQBN) \
 		-libraries $(ARDUINO_LOCAL_LIB_PATH) \
@@ -105,6 +114,7 @@ compile: dirs
 		$(VERBOSE) \
 		-build-path $(BUILD_PATH) \
 		-ide-version $(ARDUINO_IDE_VERSION) \
+		$(ARDUINO_AVR_GCC_PREFIX_PREF) \
 		examples/KeyboardioFirmware/$(SKETCH)
 	@cp $(BUILD_PATH)/$(SKETCH).hex $(HEX_FILE_PATH)
 	@cp $(BUILD_PATH)/$(SKETCH).elf $(ELF_FILE_PATH)