As far as i understood @mladen the problem here is that that there is no option to change the order in which seabios (the coreboot payload) looks for bootloader. So when an m2 device is installed it will only search for a bootloader there and you have to manually select an other devices if nothing is present on your m.2 device.
This is far from ideal but there is nothing you can do from the OS side. And it has nothing to do with the sda and sdb naming which is given by the OS and the problem lies before the OS is loaded.
So for a solution look into seaBIOS options. I’m not sure what you can do there but it might be possible to change these option if you compile your own Coreboot with custom settings. @kakaroto might have a solution for this. Or look into the coreboot thread:
Here are also some information on the topic:
Edit:
Here are detailed information about the bootorder config:
https://www.coreboot.org/SeaBIOS#Configuring_boot_order