Made 2-stage boot work.

- 1st stage (to be flashed at boot) finds 2nd stage bootloader, named 2nb_boot.bin, on the microSD card, loads it from RAM and runs it.
Error checks are flashed on the LED
- 2nd stage bootloader is just a blink for now, needs to be linked to SRAM2

see memories spreadsheet to better understand what happens.
small error on the 1st bootloader