busybox: don't disable static option

Don't change the busybox STATIC option if buildroots PREFER_STATIC_LIB option
is not set. Not to prefer static linking doesn't imply prefer dynamic linking
for all packages.

Signed-off-by: Patrick Ziegler <patrick.ziegler@fh-kl.de>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>