sconeserver: use direct imagemagick disable

Disabling sconesite image isn't enough to avoid imagemagick
autodetection, and if it's present in the host (distro) but not in the
target it gets automatically picked up. Fixes:
http://autobuild.buildroot.net/results/377/37705926f3023395c54af0532ff95f0125fbaa66/



Also add a comment about autoreconf.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>