boot/at91bootstrap3: bump to v3.7.1

Change to use the _defconfig targets of at91bootstrap3 build system
when a defconfig is used.

[Thomas: adjust to make a better use of at91bootstrap3 defconfig
mechanism.]

Signed-off-by: Angelo Compagnucci <angelo.compagnucci@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>