Also, please keep in mind Ludo's suggestion of using the first class profiles
of G-Expressions.  I'm not sure it's applicable in this case (!)--but still, it
would be nice, for a future version of raspberry efi netboot or of similar
things, to be able to do this, right from config.scm:

  (operating-system
    (bootloader
      (bootloader-configuration
        (bootloader ; field in bootloader-configuration
          (bootloader ; bootloader record constructor
            (inherit grub-efi-netboot-bootloader) ; so we have the installer
            (package ; override bootloader's package
             (profile ; create a profile
              (content
               (list (file-append firmware "/boot/")
                     (file-append u-boot-my-scb "/libexec/u-boot.bin")
                     (plain-file "config.txt"
                                 "kernel=u-boot.bin")
                     (bootloader-package grub-efi-netboot-bootloader))))) ; and put the package we've overridden back into the profile
            (target "/boot"))))

This way, maybe the procedures efi-bootloader-chain and
efi-bootloader-profile would be superfluous.

Note: I haven't used this myself yet.