To get boot devices, use cbmem, part of coreboot project.
Compile and install:
cd coreboot/util/cbmem
make
sudo make install
Run it to retrieve the boot devices:
sudo cbmem -c | grep "Searching bootorder" >> bootorder.txt
Rearrange the bootorder to your needs inside the file bootorder.txt and add it to your coreboot.rom:
sudo cbfstool coreboot.rom add -f bootorder.txt -n bootorder -t raw