build: make prefix mapping of debug information optional

Remapping the local build path in debug information makes debugging
using ./scripts/remote-gdb harder, because files no longer refer to the full
path on the build host.

For local builds, debug information does not need to be reproducible,
since it will be stripped out of packages anyway.

For buildbot builds, it makes sense to keep debug information reproducible,
since the full path is not needed (nor desired) anywhere.

Signed-off-by: Felix Fietkau <nbd@nbd.name>