build-sys: copy rootfs to another directory

build-sys: copy rootfs to another directory

We can't use the rootfs directory because it breaks out-of-tree build
and in future we want to make modifications to the fake filesystem such
as adding and removing files.

We need to call "chmod -R +w" in the resulting directory because when we
distribute the source with make dist all files will be readonly.

Fix 'make distcheck'