Following discussion in <https://issues.guix.gnu.org/37305>, it seems more
appropriate to give the parameter a more generic name that better describes
what it does.
* gnu/bootloader/grub.scm (normalize-file): Rename the
BTRFS-SUBVOLUME-FILE-NAME parameter to STORE-DIRECTORY-PREFIX, and always
assume this argument to be a string.
(eye-candy): Likewise. Default STORE-DIRECTORY-PREFIX to "".
(grub-configuration-file): Likewise.
* gnu/system.scm (operating-system-bootcfg): Adapt.
e7b86a0d88