Added flash store and load of config options.

Currently we store the config only on successful bind. As there is no
way of changing other config options at the moment.
[FlashConfig] Added a comment about code efficiency.
[Make] Clean src directory when running clean target from top level.
[Config] Do not force DSM2 mode by default.

This should make binding work with more Spektrum transmitters, that
despite being in DSM2 compatibility mode still send DSMX binding
sequence.