configs: at91sam926* use latest u-boot and mainline linux

While upgrading the configs, it proved necessary to make more space on
the dataflash for u-boot since it grew because of relocation.

dataflash is then repartitionned to make more room for u-boot and a redundant
environment.

Signed-off-by: Alexandre Belloni <alexandre.belloni@piout.net>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>