Correctly use earlephilhower.varant if variant not given, ensure USB power...

 Correctly use earlephilhower.varant if variant not given, ensure USB power macro is always there (#444)

Fixes a crash of the builder script, `board.get("build.variant", None)` will throw an exception if the `build.variant` was not found (and not return `None` as the fallback value), but an empty string works, so check against that.

USB power defines moved directly into the board files, but still ensure that the macro always exists (with a default fallback value) to not fail the build.

tools/platformio-build.py

@@ -148,10 +148,13 @@ def configure_usb_flags(cpp_defines):
         ("USB_PID", usb_pid),
         ("USB_MANUFACTURER", '\\"%s\\"' % usb_manufacturer),
         ("USB_PRODUCT", '\\"%s\\"' % usb_product),
-        ("SERIALUSB_PID", usb_pid),
-        ("USBD_MAX_POWER_MA", 250)
+        ("SERIALUSB_PID", usb_pid)
     ])
 
+    if "USBD_MAX_POWER_MA" not in env.Flatten(env.get("CPPDEFINES", [])):
+        env.Append(CPPDEFINES=[("USBD_MAX_POWER_MA", 500)])
+        print("Warning: Undefined USBD_MAX_OWER_MA, assuming 500mA")
+
     # use vidtouse and pidtouse 
     # for USB PID/VID autodetection
     hw_ids = board.get("build.hwids", [["0x2E8A", "0x00C0"]])
@@ -195,9 +198,9 @@ if not board.get("build.ldscript", ""):
 
 libs = []
 
-variant = board.get("build.arduino.earlephilhower.variant", board.get("build.variant", None))
+variant = board.get("build.arduino.earlephilhower.variant", board.get("build.variant", ""))
 
-if variant is not None:
+if variant != "":
     env.Append(CPPPATH=[
         os.path.join(FRAMEWORK_DIR, "variants", variant)
     ])