gnu: bootloader: Fix install-allwinner64-u-boot....This fixes boot breakage introduced in commit:
a65c935e29766940148d52b8116634b1e1cbcba6
Multiple files were still needed, but the filenames changed.
* gnu/bootloader/u-boot.scm (install-allwinner64-u-boot): Update to
use both SPL and u-boot files with new filenames.