and misc changes

Signed-off-by: Syrone Wong <wong.syrone@gmail.com>

- drop unused using directive
- add a comment

Signed-off-by: Syrone Wong <wong.syrone@gmail.com>

- Drop redundant code.
  I should read code carefully, mbed TLS will set operation
  when invoking mbedtls_cipher_setkey(), the last param
  const mbedtls_operation_t operation is what we want.
- minor changes

Signed-off-by: Syrone Wong <wong.syrone@gmail.com>

* Refine encryption

- Add AES-CTR, blowfish and camellia ciphers

aes-256-ctr
aes-192-ctr
aes-128-ctr
bf-cfb
camellia-128-cfb
camellia-192-cfb
camellia-256-cfb

- Merge the previous PolarSSL and MbedTLS
- Switch to MbedTLS's cipher layer functions
- Add workaround to set cipher operation(encrypt/decrypt)

Signed-off-by: Syrone Wong <wong.syrone@gmail.com>

* Add function to get MbedTLS cipher context size

and drop hard-coded ctx sizes

Signed-off-by: Syrone Wong <wong.syrone@gmail.com>