smartpower: total bikeshedding optimisation

Only work out the separator once, and reuse it from then on.

   text    data     bss     dec     hex filename
   8203     372      40    8615    21a7 smartpower.before
   8171     372      44    8587    218b smartpower.after

Signed-off-by: Phil Carmody <fatphil@asdf.org>
Signed-off-by: Aliaksei Katovich <aliaksei.katovich@gmail.com>
smartpower: squash compiler warning for a printf

The modern solution is to use z for size_t's. But that
assumes a compiler from this century. Casting always
works.

Signed-off-by: Phil Carmody <fatphil@asdf.org>
Signed-off-by: Aliaksei Katovich <aliaksei.katovich@gmail.com>
smartpower: Tart up the Makefile

Let's use some dependencies, and use shorthand for the
target and the source. Might as well enable a bit of
optimisation whilst we're there.

Signed-off-by: Phil Carmody <fatphil@asdf.org>
Signed-off-by: Aliaksei Katovich <aliaksei.katovich@gmail.com>