SERCOM config for PB10, PB11 incorrect.

[WestfW]

The various variants that define Arduino Pins for PB10 and PB11 have:

/* +------------+------------------+--------+-----------------+--------------------------------------------------------------------------------------------------------
* | Pin number | Serial2          |  PIN   | Label/Name      | Comments (* is for default peripheral in use)
* +------------+------------------+--------+-----------------+--------------------------------------------------------------------------------------------------------
* | 30         |                  |  PB11  | EXT2_13/EXT3_13 | EIC/EXTINT[11] *SERCOM4/PAD[3]  TC5/WO[1]  TCC0/WO[5]  I2S/SCK[1]  GCLK_IO[5]
* | 31         |                  |  PB10  | EXT2_14/EXT3_14 | EIC/EXTINT[10] *SERCOM4/PAD[2]  TC5/WO[0]  TCC0/WO[4]  I2S/MCK[1]  GCLK_IO[4]
* +------------+------------------+--------+-----------------+--------------------------------------------------------------------------------------------------------
*/
 { PORTB, 11, PIO_SERCOM, PIN_ATTR_DIGITAL, No_ADC_Channel, NOT_ON_PWM, NOT_ON_TIMER, EXTERNAL_INT_11 }, // RX: SERCOM4/PAD[3]
 { PORTB, 10, PIO_SERCOM, PIN_ATTR_DIGITAL, No_ADC_Channel, NOT_ON_PWM, NOT_ON_TIMER, EXTERNAL_INT_10 }, // TX: SERCOM4/PAD[2]
/*

In fact, PB10 and PB11 do not have a SERCOM pinmux setting, only a SERCOM_ALT pinmux (which does provide Sercom4 pads as the comment documents.

This prevents the typical setup of an additional Serial interface on those pins.