buildsys: fix make release target

buildsys: fix make release target

According to git man page the syntax for git archive should have
the <tree-ish> as last argument. Indeed using git 1.5.5.6 the tarball
was not properly created. Newer version (i.e. 1.6.3.3) works fine even if
HEAD comes before other option. For backward compatibility use a syntax
that is fine with older git.

Signed-off-by: Carmelo Amoroso <carmelo.amoroso@st.com>