have zlib respect BR2_PREFER_STATIC_LIB

The ARM uClinux BFLT format apparently doesn't support shared libs nor
-fPIC.  So let's distinguish between shared and non shared builds by
having split rules for libz.a and libz.so based on BR2_PREFER_STATIC_LIB.

Signed-off-by: Nicolas Pitre <nico@cam.org>