config-mode, mesh-vpn: refactor key generation

Previously, the config-mode was responsible for generating the fastd
secret. This patch sets the default secret to "generate" causing a
secret to be generated on its first use (e.g. show_key or start).

This also changes the info page (in Expert Mode) to show "n/a" when the
public key is not yet available.
Merge pull request #53 from freifunk-gluon/fastd-key-generate

config-mode, mesh-vpn: refactor key generation