at91bootstrap: do not use specific branch
at91bootstrap: do not use specific branch

Branch "at91bootstrap-3.x-python3" does not exists anymore since
master includes python3 support, so use master instead.

Fixes:
 - http://51.75.135.20:8080/job/sama5d27-som1-ek-sd/75/console

Signed-off-by: Pierre-Jean Texier <188dcb29e125b35642aafd9b1cc33149502c1da3@koncepto.io>

linux4sam/meta-atmel,linux4sam/meta-atmel,linux4sam/meta-atmel,linux4sam/meta-atmel,linux4sam/meta-atmel