at91sam* fix DEFAULT_TUNE

Machines with an ARMv5TE class cpu are expected to produce packages with 'armv5te' as architecture, the current at91 configs break that by using 'arm926ejs' as architecture. This change has no performance impact is is purely there to unbreak DISTROs supporting multiple ARMv5TE machines.

This fixes issue #6

Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>