bug#37286: Make a faster method to list supported boards.

[Jesse Gibbons <jgibbons2357@gmail.com>]

There should be a faster way to list the supported boards with custom
versions of U-Boot.

From manual:
Many ARM boards require a specific variant of the U-Boot
(https://www.denx.de/wiki/U-Boot/) bootloader.

   If you build a disk image and the bootloader is not available
otherwise (on another boot drive etc), it’s advisable to build an image
that includes the bootloader, specifically:

     guix system disk-image --system=armhf-linux -e '((@ (gnu system
install) os-with-u-boot) (@ (gnu system install) installation-os) "A20-
OLinuXino-Lime2")'

   ‘A20-OLinuXino-Lime2’ is the name of the board.  If you specify an
invalid board, a list of possible boards will be printed.



I think an example of an invalid board is "dne-board", which (last I
checked) does not exist and probably never will exist. If I run the
following command:

guix system disk-image --system=armhf-linux -e '((@ (gnu system
install) os-with-u-boot) (@ (gnu system install) installation-os) "dne-
board")'

guix tries to build an entire system. It doesn't look like it checks if
"dne-board" is a valid board until it is building the image. It takes
hours to cross-compile the kernel, and even longer if the kernel needs
to be deblobbed. I have not yet been able to cross-compile any system
using this method.