at91bootstrap: switch to the official version

The Buildroot makefile was fetching and building the special
AT91Bootstrap of Ulf, which is not the Atmel official version. While
Ulf's variant has a better configuration/build system, the Atmel
version, as officially supported, is probably a better choice for the
future.

The Atmel version only needed a small tweak to work with EABI
toolchains.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>