pcengines/apu: expose GRUB over serial

Prior to this commit, an apu's boot would go through stages of varying visibility:
1. BIOS: exposed to serial
2. GRUB: not exposed to serial
3. Linux: exposed to serial

This commit changes (2) to be visible over serial, so the entire boot is visible over serial.

Tested on nixos 18.09 on a https://www.pcengines.ch/apu1d.htm