auc: compare versions using dpkg/opkg's verrevcmp

Using strcmp() to compare a version string doesn't work well.
Use verrevcmp() function from opkg instead.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
(cherry picked from commit 794cf3ac,
as it was previously skipped also switch PKG_REVISION back to manual
to keep it in sync with master branch)