package.bbclass: correct check for /build in copydebugsources()

package.bbclass: correct check for /build in copydebugsources()

Newly introduced kirkstone-only commit
https://git.openembedded.org/openembedded-core/commit/?h=kirkstone&id=80839835ec9fcb63069289225a3c1af257ffdef7
broke builds with externalsrc in Gitlab-CI.
This is yocto-4.0.9 regression.

It checks if directory starts with "build" instead of
if checking if it equals to "build".
Gitlab-CI uses directory "/builds" which matches the check
but directory /build does not exist, only /builds.
After successful check it tries to move this non-existent
directory which does not exists and thus do_package fails.

Signed-off-by: Peter Marko <peter.marko@siemens.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>