classes/buildhistory: fix failures collecting output signatures

classes/buildhistory: fix failures collecting output signatures

It's possible for tasks to stage symlinks that point to non-existent
files; an example is ncurses-native.do_populate_sysroot. There wasn't
any error checking here so this broke the build when "task" was included
in BUILDHISTORY_FEATURES. In any case we shouldn't be following symlinks
and getting the sha256sum of the link target - we need concern ourselves
only with the target path, so check if the file is a link and sha256 the
target path instead if it is. If it's neither a regular file nor a
symlink (perhaps a pipe or a device), just skip it.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>