bitbake/utils: Convert vercmp_string() to use vercmp internally

bitbake/utils: Convert vercmp_string() to use vercmp internally

Having two different version comparision algorithms in bitbake has never seemed
like a sensible idea. Worryingly, they also return different results to each other.

The vercmp_string API is relatively unused with no users in OE-Core or BitBake
itself for example. This patch converts it to use vercmp internalls, bringing
consitency to the comparisions which is easy now we have other recently added
functions. Yes, this changes behaviour but in this case I'd prefer we were
consistent than having two different comparisions.

(Bitbake rev: a569c816e016447d60624c59a750709d59a0f455)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>