image_verify: Append .sig instead of replace

For the signature files, the code was using replace_extension() to
replace the extension, it works for files without extension, e.g.
for image-bmc it gets image-bmc.sig.

But for files with existing extensions, e.g. bios.bin, it gets bios.sig
instead of bios.bin.sig, where the latter is valid.

So use fs::path::operator += to append the `.sig` string to get the
expected signature file name.

Signed-off-by: Lei YU <yulei.sh@bytedance.com>
Change-Id: Ia753aaf1e8ae2be285c700efb13770e02166c6d2
1 file changed