build: create tarball without self-referential hard links

The current method of creating the tarball, which is to the hard-link
the source directory to the target directory, results in self-referential
hardlinks which can be observed using tar xf.

This patch resolves this by using an intermediate tarball, held in memory,
which collects files to be distributed. This is then unpacked in the target
directory which is finally packed into the distribution tarball, a file.

Signed-off-by: Simon Horman <horms@verge.net.au>