Fix Adafruit board compile errors.

Former-commit-id: f345c3c

Author: h2zero

variants/circuitplayground_nrf52840/variant.h

@@ -97,6 +97,7 @@ static const uint8_t A9  = PIN_A9 ;
  */
 #define PIN_SERIAL1_RX       (0)
 #define PIN_SERIAL1_TX       (1)
+#define USB_CDC_DEFAULT_SERIAL 1
 
 /*
  * SPI Interfaces

variants/clue_nrf52840/variant.h

@@ -92,6 +92,7 @@ static const uint8_t A7  = PIN_A7 ;
  */
 #define PIN_SERIAL1_RX       (0)
 #define PIN_SERIAL1_TX       (1)
+#define USB_CDC_DEFAULT_SERIAL 1
 
 /*
  * SPI Interfaces

variants/feather_nrf52840_express/variant.h

@@ -96,6 +96,7 @@ static const uint8_t AREF = PIN_AREF;
  */
 #define PIN_SERIAL1_RX       (1)
 #define PIN_SERIAL1_TX       (0)
+#define USB_CDC_DEFAULT_SERIAL 1
 
 /*
  * SPI Interfaces

variants/feather_nrf52840_sense/variant.h

@@ -100,6 +100,7 @@ static const uint8_t AREF = PIN_AREF;
  */
 #define PIN_SERIAL1_RX       (1)
 #define PIN_SERIAL1_TX       (0)
+#define USB_CDC_DEFAULT_SERIAL 1
 
 /*
  * SPI Interfaces

variants/itsybitsy_nrf52840_express/variant.h

@@ -91,6 +91,7 @@ static const uint8_t A6  = PIN_A6 ;
  */
 #define PIN_SERIAL1_RX       (0)
 #define PIN_SERIAL1_TX       (1)
+#define USB_CDC_DEFAULT_SERIAL 1
 
 /*
  * SPI Interfaces