staging: Handle overlapping files for build-sysroot
The use of bb.fatal means build-sysroots fails as soon as multiple gdb's are
built with overlapping files, or multiple recipes with overlapping headers
exist.
Change the fatal call into an exception which we can then trap. Also
avoid trying to call readlink on something with isn't a symlink.
This allows build-sysroots to work better under various scenarios.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
