Add SAMD21 and SAMD51 support for CircuitPython.

The ProtoThreads style subtasks were removed because it led to
extremely unclear control flow. RTOSes can be used if threading is
needed.

Also added some additional functionality to MSC to support dynamic
LUNs and read-only LUNs.

src/class/msc/msc.h

@@ -279,11 +279,13 @@ typedef struct ATTR_PACKED
 
 TU_VERIFY_STATIC( sizeof(scsi_mode_sense6_t) == 6, "size is not correct");
 
+// This is only a Mode parameter header(6).
 typedef struct ATTR_PACKED
 {
   uint8_t data_len;
   uint8_t medium_type;
-  uint8_t device_specific_para;
+  bool write_protected : 1;
+  uint8_t reserved : 7;
   uint8_t block_descriptor_len;
 } scsi_mode_sense6_resp_t;