uencrypt: split common and library-specific code

uencrypt: split common and library-specific code

This splits the code in 4 files:
 - uencrypt.h
 - uencrypt.c - main program
 - uencrypt-openssl.c - OpenSSL/wolfSSL implementation
 - uencrypt-mbedtls - mbedTLS implementation

Other changes, accounting for ~400 bytes increase in ipk size:
 - more error condition checking and reporting,
 - hide key and iv command line arguments

Signed-off-by: Eneas U de Queiroz <cotequeiroz@gmail.com>