efi: Renames in handle_cmdline_files() to complete generalization.

Rename variables to be not initrd specific, as now the function
loads arbitrary files.  This change is exclusively renames
and comment changes to reflect the generalization.

Signed-off-by: Roy Franz <roy.franz@linaro.org>
Acked-by: Mark Salter <msalter@redhat.com>
Reviewed-by: Grant Likely <grant.likely@linaro.org>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>