tls: add support for TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 cipher

tls: add support for TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 cipher

function                                             old     new   delta
xwrite_encrypted                                     209     605    +396
GHASH                                                  -     395    +395
aes_encrypt_1                                          -     382    +382
GMULT                                                  -     192    +192
tls_xread_record                                     489     659    +170
aes_encrypt_one_block                                  -      65     +65
aesgcm_setkey                                          -      58     +58
FlattenSzInBits                                        -      52     +52
tls_handshake                                       1890    1941     +51
xwrite_and_update_handshake_hash                      46      81     +35
xorbuf                                                 -      24     +24
aes_setkey                                             -      16     +16
psRsaEncryptPub                                      413     421      +8
stty_main                                           1221    1227      +6
ssl_client_main                                      138     143      +5
next_token                                           841     845      +4
spawn_ssl_client                                     218     219      +1
volume_id_probe_hfs_hfsplus                          564     563      -1
read_package_field                                   232     230      -2
i2cdetect_main                                       674     672      -2
fail_hunk                                            139     136      -3
parse_expr                                           891     883      -8
curve25519                                           802     793      -9
aes_cbc_decrypt                                      971     958     -13
xwrite_handshake_record                               43       -     -43
aes_cbc_encrypt                                      644     172    -472
------------------------------------------------------------------------------
(add/remove: 9/1 grow/shrink: 9/8 up/down: 1860/-553)        Total: 1307 bytes

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