Don't truncate MAC key when truncated HMAC is negotiated
Don't truncate MAC key when truncated HMAC is negotiated

The truncated HMAC extension as described in
https://tools.ietf.org/html/rfc6066.html#section-7 specifies that when truncated
HMAC is used, only the HMAC output should be truncated, while the HMAC key
generation stays unmodified. This commit fixes Mbed TLS's behavior of also
truncating the key, potentially leading to compatibility issues with peers
running other stacks than Mbed TLS.

Details:
The keys for the MAC are pieces of the keyblock that's generated from the
master secret in `mbedtls_ssl_derive_keys` through the PRF, their size being
specified as the size of the digest used for the MAC, regardless of whether
truncated HMAC is enabled or not.

             /----- MD size ------\ /------- MD size ----\
Keyblock    +----------------------+----------------------+------------------+---
now         |     MAC enc key      |      MAC dec key     |     Enc key      |  ...
(correct)   +----------------------+----------------------+------------------+---

In the previous code, when truncated HMAC was enabled, the HMAC keys
were truncated to 10 bytes:

             /-10 bytes-\  /-10 bytes-\
Keyblock    +-------------+-------------+------------------+---
previously  | MAC enc key | MAC dec key |     Enc key      |  ...
(wrong)     +-------------+-------------+------------------+---

The reason for this was that a single variable `transform->maclen` was used for
both the keysize and the size of the final MAC, and its value was reduced from
the MD size to 10 bytes in case truncated HMAC was negotiated.

This commit fixes this by introducing a temporary variable `mac_key_len` which
permanently holds the MD size irrespective of the presence of truncated HMAC,
and using this temporary to obtain the MAC key chunks from the keyblock.

ARMmbed/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls