utils: add stoken

stoken is a tokencode generator compatible with RSA SecurID
128-bit (AES). stoken supports libtomcrypt and libnettle, we build
against nettle by default since tomcrypt is not packaged by OpenWrt

Signed-off-by: Florian Fainelli <florian@openwrt.org>
openconnect: add an option to support stoken

Add a new build configuration option for openconnect and let it link
against libstoken if instructed to. Two new uci configuration variables
are introduced: "token_mode" and "token_secret" to allow openconnect to
use those.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Merge pull request #595 from ffainelli/stoken

Add stoken and update openconnect to use it