package.bbclase: Add check for /build in copydebugsources()

This is needed when the SDK or eSDK is installed in a /build top level
directory as it conflicts with the build directory within the existing
/usr/src/debug/build (which is really a link). Rename it and then do the
copy, this is not an issue with master currently due to some other
changes that occurred in master.

Fixes: [YOCTO #15026]

Signed-off-by: Saul Wold <saul.wold@windriver.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>