uencrypt: add support for mbedtls

uencrypt: add support for mbedtls

This commit includes some additional changes:
 - better handling of iv and keys in openssl/wolfssl variants
 - fix compiler warnings and whitespace
 - build all 3 variants as separate packages
 - adjust the new package name in targets' DEVICE_PACKAGES
 - remove PKG_FLAGS:=nonshared

[Beeline SmartBox Flash - OK]
Tested-by: Mikhail Zhilkin <[email protected]>
[after test: replaced a hardcoded IV size of 16 by cipher_info->iv_size]
Signed-off-by: Eneas U de Queiroz <[email protected]>