signlk: always invoke make for signer/signlk

Instead of checking if signer/signlk exists , let's unconditionally invoke the
makefile and let the makefile decide if anything need to be rebuilt. This makes
the script a bit more clear, and avoid issues if/when an old version of
signer/signlk might exist.

Also, keep the make output visible, in case something goes bad.

Signed-off-by: Nicolas Dechesne <nicolas.dechesne@linaro.org>