Add usb-endpoint-count checking

.. however, the number of endpoints is only set for SAMD (8).
Other ports need to set the value.  Otherwise, the build will show
the message
```
Unable to check whether maximum number of endpoints is respected
```

Author: jepler

ports/atmel-samd/mpconfigport.mk

@@ -91,3 +91,5 @@ endif # samd51
 INTERNAL_LIBM = 1
 
 USB_SERIAL_NUMBER_LENGTH = 32
+
+USB_NUM_EP = 8

supervisor/supervisor.mk

@@ -171,6 +171,10 @@ ifndef USB_MIDI_EP_NUM_IN
 USB_MIDI_EP_NUM_IN = 0
 endif
 
+ifndef USB_NUM_EP
+USB_NUM_EP = 0
+endif
+
 USB_DESCRIPTOR_ARGS = \
 	--manufacturer $(USB_MANUFACTURER)\
 	--product $(USB_PRODUCT)\
@@ -180,6 +184,7 @@ USB_DESCRIPTOR_ARGS = \
 	--interface_name $(USB_INTERFACE_NAME)\
 	--devices $(USB_DEVICES)\
 	--hid_devices $(USB_HID_DEVICES)\
+	--max_ep $(USB_NUM_EP) \
 	--cdc_ep_num_notification $(USB_CDC_EP_NUM_NOTIFICATION)\
 	--cdc_ep_num_data_out $(USB_CDC_EP_NUM_DATA_OUT)\
 	--cdc_ep_num_data_in $(USB_CDC_EP_NUM_DATA_IN)\

tools/gen_usb_descriptor.py

@@ -62,6 +62,8 @@
                     help='endpoint number of MIDI OUT')
 parser.add_argument('--midi_ep_num_in', type=int, default=0,
                     help='endpoint number of MIDI IN')
+parser.add_argument('--max_ep', type=int, default=0,
+                    help='total number of endpoints available')
 parser.add_argument('--output_c_file', type=argparse.FileType('w', encoding='UTF-8'), required=True)
 parser.add_argument('--output_h_file', type=argparse.FileType('w', encoding='UTF-8'), required=True)
 
@@ -376,6 +378,15 @@ def strings_in_order(cls):
 # interface cross-references.
 interfaces = util.join_interfaces(interfaces_to_join, renumber_endpoints=args.renumber_endpoints)
 
+if args.max_ep != 0:
+    for interface in interfaces:
+        for subdescriptor in interface.subdescriptors:
+            endpoint_address = getattr(subdescriptor, 'bEndpointAddress', 0) & 0x7f
+            if endpoint_address > args.max_ep:
+                raise ValueError("Endpoint address %d of %s may not exceed %d" % (endpoint_address & 0x7f, interface.description, args.max_ep))
+else:
+    print("Unable to check whether maximum number of endpoints is respected", file=sys.stderr)
+
 # Now adjust the CDC interface cross-references.
 
 cdc_union.bMasterInterface = cdc_comm_interface.bInterfaceNumber