nacl scripts: include common header and rename build_nacl.sh

Add a uniform header comment to the two scripts as present in
other scripts in this repository as well. Also rename build_nacl.sh
into nacl_build.sh to stay conform in naming. For this, also update
all mentions of build_nacl.sh to nacl_build.sh.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>