openssl: configure engines with uci

openssl: configure engines with uci

This uses uci to configure engines, by generating a list of enabled
engines in /var/etc/ssl/engines.cnf from engines configured in
/etc/config/openssl:

    config engine 'devcrypto'
            option enabled '1'

Currently the only options implemented are 'enabled', which defaults to
true and enables the named engine, and the 'force' option, that enables
the engine even if the init script thinks the engine does not exist.

The existence test is to check for either a configuration file
/etc/ssl/engines.cnf.d/%ENGINE%.cnf, or a shared object file
/usr/lib/engines-1.1/%ENGINE%.so.

The engine list is generated by an init script which is set to run after
'log' because it informs the engines being enabled or skipped.  It
should run before any service using OpenSSL as the crypto library,
otherwise the service will not use any engine.

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