console: Move poor-man's atoi() into string.h

console: Move poor-man's atoi() into string.h

vtxprintf.c seems to have been written before string.h was as fleshed
out as it is today -- this patch removes some custom implementation of
stuff we now have globally. It also makes the skip_atoi() function
globally available, because I need it somewhere else, and while we maybe
don't want a huge fully-featured string parsing library in coreboot,
being able to parse an integer is occasionally useful.

Change-Id: Iecb2b970aecfc768540d2bf8b3023445f54853a4
Signed-off-by: Julius Werner <jwerner@chromium.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/32858
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Alex Thiessen <alex.thiessen.de+coreboot@gmail.com>