The paths for the kernel and the initrd in the BLS snippets are
meant to be relative to the root of the filesystem they are on.
The current code assumes that kernel and initrd are installed
under '/boot' and that '/boot' is on the root file system and
thus all paths get fixed up to start with '/boot/…'. But the
'/boot' directory can be on a separate partition and thus file
system, and then paths need to be relative to that and should
be fixed up with '/…'. Introduce a new option 'prefix' that
can be used to manually specify the prefix after the fixup,
defaulting to '/boot' for backwards compatibility.
NB: The canonical Boot Loader Specification[1] requires that
a separate partition is used boot related files and it will
be mounted at '/boot' (or '/efi').
[1] https://systemd.io/BOOT_LOADER_SPECIFICATION/
abfc4d6b5a