Rework encryption extension

[hmatuschek]

There is actually a more complex structure of the encryption keys possible. There are three encryption methods

• basic -- Some unsave scrambling using variable sized keys
• enhanced -- RC4, 40bit (probably also variable in size)
• advanced -- AES, variable sized keys

The keys sizes are then device specific?!?

Todo list:

• [x] Make keys sizes variable
• [x] Add enhanced key type
• [x] Add key-size check per supported device (RadioLimits).
• [x] Fix documentation.

[hmatuschek]

Merged into devel.