at91bootstrap 3.6.2: allow custom targets
at91bootstrap 3.6.2: allow custom targets

This way a MACHINE.conf can set it to non-default values.

Signed-off-by: Koen Kooi <42e3d96910f922bdd7dd1805fe3f8aff634f391d@dominion.thruhere.net>

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