Pinebook pro build

[Grant Wilson II <grantm.wilsonii@gmail.com>]

I am attempting to build a bare bones image for the pinebook pro. Either
using `guix system image` or `guix system init`. I have noodled around with
many approaches but all have failed.
At the moment I am trying to cross compile this from an x86_64 system.
The following build command is failing and I am not sure why.

`guix build --target=aarch64-linux-gnu -e "(@ (gnu packages bootloaders)
u-boot-pinebook-pro-rk3399)"`

"
...
starting phase `set-environment'
Backtrace:
           9 (primitive-load "/gnu/store/720rvsfh0wgq2dik4jvi3m357hr…")
In ice-9/eval.scm:
   191:35  8 (_ #f)
In guix/build/gnu-build-system.scm:
    838:2  7 (gnu-build #:source _ #:outputs _ #:inputs _ #:phases . #)
In ice-9/boot-9.scm:
  1736:10  6 (with-exception-handler _ _ #:unwind? _ # _)
In srfi/srfi-1.scm:
   857:16  5 (every1 #<procedure 7ffff19d7580 at guix/build/gnu-bui…> …)
In guix/build/gnu-build-system.scm:
   847:30  4 (_ _)
In ice-9/eval.scm:
    619:8  3 (_ #(#(#<directory (guile-user) 7ffff1baff00>) (# # …)))
    159:9  2 (_ #(#(#<directory (guile-user) 7ffff1baff00>) (# # …)))
In unknown file:
           1 (string-append #f "/bl31.elf")
In ice-9/boot-9.scm:
  1669:16  0 (raise-exception _ #:continuable? _)

ice-9/boot-9.scm:1669:16: In procedure raise-exception:
In procedure string-append: Wrong type (expecting string): #f
builder for
`/gnu/store/rlfcsy66wi25sr75ll6b2gbx0j4sfxhs-u-boot-pinebook-pro-rk3399-2021.04.drv'
failed with exit code 1
build of
/gnu/store/rlfcsy66wi25sr75ll6b2gbx0j4sfxhs-u-boot-pinebook-pro-rk3399-2021.04.drv
failed
View build log at
'/var/log/guix/drvs/rl/fcsy66wi25sr75ll6b2gbx0j4sfxhs-u-boot-pinebook-pro-rk3399-2021.04.drv.bz2'.
guix build: error: build of
`/gnu/store/rlfcsy66wi25sr75ll6b2gbx0j4sfxhs-u-boot-pinebook-pro-rk3399-2021.04.drv'
failed
"

I am curious about this specific issue, but any suggestions on how to
accomplish my main goal of building any guixsd image for a pinebook-pro are
welcome.

Thank you!