After some discussion, we found out that "tools" has the four first
letters identical to the "toolchain" subfolder, which makes it a bit
unpractical with tab-completion. So, this commit renames "tools" to
"utils", which is more tab-completion-friendly.

This has been discussed with Arnout and Yann.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>