dropbear: use config_get_bool enable

dropbear: use config_get_bool enable

The config_get_bool also works with on/off, yes/no, true/false.
Add 'main' section name. This will make it easier to change settings from uci.
Add a link to documentation.

Signed-off-by: Sergey Ponomarev <stokito@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/15579
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
(cherry picked from commit 4511fa4b30f73185597990ce563bbf3f96385292)
Link: https://github.com/openwrt/openwrt/pull/17097
Signed-off-by: Petr Štetiar <ynezz@true.cz>