release: remove manual build files from release tarballs

The Buildroot release tarballs inadvertently contain a build/docs directory,
containing the manual sources, the generated lists, and manual.text and
manual.pdf (but excluding manual.html).

This directory is populated as $(BUILD_DIR) (==$(O)/build), while O is
set explicitly from the release target to a subdirectory
buildroot-xxxx.yy-git/ which was populated with 'git archive'.

Since the generated manuals are available in docs/manual, which is
also referred to from the README, the build directory is not needed and
should be removed from the release tarball.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>