efi_loader: Increase path string to 32 characters

Whenever we want to tell our payload about a path, we limit ourselves
to a reasonable amount of characters. So far we only passed in device
names - exceeding 16 chars was unlikely there.

However by now we also pass real file path information, so let's increase
the limit to 32 characters. That way common paths like "boot/efi/bootaa64.efi"
fit just fine.

Signed-off-by: Alexander Graf <agraf@suse.de>