tls: introduce FEATURE_TLS_SHA1 to make SHA1 code optional

tls: introduce FEATURE_TLS_SHA1 to make SHA1 code optional

When disabled:

function                                             old     new   delta
xwrite_encrypted                                     580     579      -1
prf_hmac_sha256                                      222     217      -5
hmac_begin                                           158     149      -9
static.ciphers                                        32      20     -12
tls_handshake                                       2115    2095     -20
hmac                                                  87      61     -26
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/6 up/down: 0/-73)             Total: -73 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>