fix(st): remove extra chars from dtc version

In some implementations of dtc tool (e.g. with yocto), there can be a 'v'
at the beginning of the version, and a '+' at the end. Just keep numbers
then, with a grep -o.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: I180e97ab75ba3e5ceacb4b1961a1f22788b428a3