Allow usage of either SD card slot

The Sabrelite has two SD card slots, one for micro-sd one for full-size SD. The installer simply assumed the full SD card slot was used (slot 0) and then didn't bother check whethering it actually succeeded in loading a u-boot binary before writing the SPI flash.

This is, for obvious reasons, not great. Instead lets scan all SD cards slots for a card with the required file on it (so either slot can be used) and check if loading succeeds before flashing the SPI.

Signed-off-by: Sjoerd Simons sjoerd.simons@collabora.co.uk