libcurl: drop polarssl support

As of curl 7.36.0 it doesn't support polarssl < 1.3 any longer. Fixes:
http://autobuild.buildroot.net/results/d82/d82c3618e9dde3da7e36ba2b58545a9a8de5e442/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>