On 52bd496, we stopped to rm -rf debian/ on build_deb.sh, since now we have
prebuilt debian/ directory.
However, it might cause .deb build error when we modified debian package source,
since it never cleanup.
To prevent build error, we need to cleanup build/debian on reloc/build_deb.sh,
before extracting contents from relocatable package.
78c3b7627f