Instead of always returning 0 in efivarfs_file_read(), even when we
fail to successfully read the variable, convert the EFI status to
something meaningful and return that to the caller. This way the user
will have some hint as to why the read failed.
Acked-by: NJeremy Kerr <jeremy.kerr@canonical.com>
Signed-off-by: NMatt Fleming <matt.fleming@intel.com>