Adds definition for WIRE_INTERFACES_COUNT #1182 (#1921)

* Adds definition for WIRE_INTERFACES_COUNT so those libraries which rely on it can detect and use...

Wire1. e.g. u8g2 will not use Wire1 unless this is (a) defined and (b) >1. This constant is defined
on other cores, e.g. for SAMD based boards.

variants/generic/common.h

@@ -7,6 +7,7 @@
 #define NUM_ANALOG_INPUTS   (4u)
 #define NUM_ANALOG_OUTPUTS  (0u)
 #define ADC_RESOLUTION      (12u)
+#define WIRE_INTERFACES_COUNT (WIRE_HOWMANY)
 
 #ifdef PIN_LED
 #define LED_BUILTIN PIN_LED