Simplify rendering of Kconfig variable doc

[carlescufi]

Currently Kconfig options are rendered in an excessively verbose way:

http://docs.zephyrproject.org/reference/kconfig/CONFIG_NET_DEBUG_L2_IEEE802154.html#cmdoption-arg-config-net-debug-l2-ieee802154

Propose to reduce that to smaller set of variables such as:

• Title
• Help text
• Dependencies
• Value
• File defined