curl: Remove PolarSSL and adjust default to mbedTLS

luci-ssl has already made the switch since mainline support for PolarSSL is
almost over (2016).

Signed-off-by: Rosen Penev <rosenp@gmail.com>

package/network/utils/curl/patches/310-mbedtls-disable-runtime-version-check.patch

@@ -0,0 +1,11 @@
+--- a/lib/vtls/mbedtls.c
++++ b/lib/vtls/mbedtls.c
+@@ -724,7 +724,7 @@ void Curl_mbedtls_session_free(void *ptr
+ 
+ size_t Curl_mbedtls_version(char *buffer, size_t size)
+ {
+-  unsigned int version = mbedtls_version_get_number();
++  unsigned int version = MBEDTLS_VERSION_NUMBER;
+   return snprintf(buffer, size, "mbedTLS/%d.%d.%d", version>>24,
+                   (version>>16)&0xff, (version>>8)&0xff);
+ }