sstate: Account for reserved characters when shortening sstate filenames

Previously, when shortening sstate filenames, the reserved
characters for .siginfo were not considered, when siginfo=False,
resulting in differently shortend filenames for the sstate and siginfo
files. With this change, the filenames of the truncated sstate and
siginfo files have the same basename, just as is already the case for
untruncated filenames.

Making sure that the .siginfo files always have the filename of the
corresponding sstate file plus its .siginfo suffix, also when being
truncated, makes it easier to manage the sstate cache and an sstate
mirror outside of Bitbake/Yocto.

Signed-off-by: Manuel Leonhardt <mleonhardt@arri.de>
Cc: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>