bug#65982: Pinebook Pro image broken

[Gabriel Wicki]

Trying to set up my Pinebook Pro with my favorite distribution i found
out that the usual way (invoking `guix system image
gnu/system/images/pinebook-pro.scm` to build an image which is then dd'd
to a micro SD card) does not yield a working result.

Doing the same with guix time-machine on commit
c9c195936d128465030bc05f8c319367602c93b5 generates a bootable image.

Since building with a current checkout does not yield a bootable image
(the pinebooks gracefully falls back to boot from the eMMC memory with
its pre-installed Manjaro) I guess the bootloader broke.