package/mbedtls: fix zlib support

package/mbedtls: fix zlib support

To enable compression support using zlib it is necessary to uncomment
the define for MBEDTLS_ZLIB_SUPPORT in config.h [1].

Note, that enabling TLS compression may make mbedTLS vulnerable to the
CRIME attack [1]. It should not be enabled unless is is sure CRIME and
similar attacks are not applicable to the particulare situation.

As zlib is probably enabled in most systems, maybe it is best to make
the compression support a user choice and add the warning from [1]?

[1] https://tls.mbed.org/kb/how-to/deflate-compression-in-ssl-tls

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>