openvpn: disable CBC record splitting in PolarSSL/mbedTLS (#19101)

OpenVPN assumes that its control channel messages are sent and received
unfragmented, this assumption is broken when CBC record splitting is
enabled in mbedTLS.

The record splitting is intended as countermeasure against BEAST attacks
which do not apply to OpenVPN, therefore we simply disable it until
upstream OpenVPN gains the ability to process fragmented control
messages.

Disabling the splitting also works around a (not remotely triggerable)
segmentation fault in mbedTLS.

References:

 * https://dev.openwrt.org/ticket/19101
 * https://community.openvpn.net/openvpn/ticket/524
 * https://github.com/ARMmbed/mbedtls/pull/185



Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>

SVN-Revision: 45602