/var/lib/misc/random-seed has served that role for years, I'm only
advocating loading it earlier in the boot process. It's *much* harder
to guess the state of random-seed than the dtb or mac address(es)...
If the bootloader is willing to reach into the file system, which means (a) having a minimal file system layer, like Grub does, and (b) can find the block device where the file is found, that's a perfectly *fine* implementation. I'm not sure mobile handset vendors will be all that psyched into either using or replicating all of Grub's functionality so it could do that, though....