Automated bootloader testing in LAVA

Bootloader testing currently has to be performed manually, because the arm boards we have typically have a preset location that they always boot from. For automated testing, we keep a recovery image that boots from the default boot partition, so if the bootloader gets updated, we are still booting from the old bootloader.

One recent possibility for making some progress on this was raised by the ARM Landing Team. The vexpress boards can be made to boot from other locations, so we may be able to use this to at least get bootloader testing for vexpress. We should also revisit the viability of other approaches for boards that do not have this capability, such as jtag and custom-built devices that have been discussed previously.