fwtool: do not omit final 16 byte when image does not contain signature

fwtool: do not omit final 16 byte when image does not contain signature

The fwutil command will interpret the final 16 byte of a given firmware
image files as "struct fwimage_trailer".

In case these bytes do look like a valid trailer, we must ensure that we
print them out along with the remainder of the image to not accidentally
truncate non-trailer-images by 16 bytes when they're piped through fwtool,
e.g. as part of an image verification command sequence.

Some command sequences pipe images through fwtool in order to strip any
possible metadata, certificate or signature trailers and do not expect
bare images without any of that metadata to get truncated as other non-
fwtool specific metadata is expected at the end of the file, e.g. an
information block with an md5sum in case of the combined image format.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>