luci-base: add uci.get_bool to allow cleanup of app code

luci-base: add uci.get_bool to allow cleanup of app code

Any number of apps read boolean values from configuration files, then use
various inconsistent means for checking truth values.  The get_bool function
allows app authors to fetch the value without regard for how it is represented
in the config file.

For example, this

    let enabled = uci.get('system', 'ntp', 'enable_server');
    if (enabled == '1') ...

could become the more natural

    let enabled = uci.get_bool('system', 'ntp', 'enable_server');
    if (enabled) ...

Signed-off-by: Eric Fahlgren <[email protected]>