package/polarssl: fix static link

- disable shared object build when BR2_PREFER_STATIC_LIB is set
- patch the CMake code for handling static/shared object build using
  standard CMake flags, instead of the ucstom ones.

Fixes:
  http://autobuild.buildroot.net/results/754/754947d2a77a4dbe91057d8ce64fc4996e716ece/



Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>