at91bootstrap: use 3.7.1 revision for all boards
at91bootstrap: use 3.7.1 revision for all boards

Signed-off-by: Nicolas Ferre <45b517be0e5ec6dc3a39715767c1d18e090d39b4@atmel.com>

linux4sam/meta-atmel,alexandrebelloni/meta-atmel,linux4sam/meta-atmel,koenkooi/meta-atmel,alexandrebelloni/meta-atmel,linux4sam/meta-atmel,koenkooi/meta-atmel,linux4sam/meta-atmel,kraj/meta-atmel,linux4sam/meta-atmel,kraj/meta-atmel,alexandrebelloni/meta-atmel,koenkooi/meta-atmel,kraj/meta-atmel