Ugh... The whole `''${devtype} ''${devnum}:''${bootpart}` thing was
cargo-culted from other u-boot scripts as "the way to go" to re-use the
information set by the generic distro boot...
EXCEPT that it doesn't work since 2018.
13dd6665ed
They are not environment variables since that change. So any of those
scripts end up working *by sheer luck* since it would end up booting
from the first device's first partition.
Ugh ugh ugh...
There is one **major** difference with the choice: We are now selecting
on a partition's label, rather than booting whatever is deemed bootable.
This assumes the upcoming change where we are using GPT rather than MBR.
But still, this is compatible with the default expectation from U-Boot
by falling back to the "bootable" attribute.
