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 <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/15579
Signed-off-by: Hauke Mehrtens <[email protected]>
(cherry picked from commit 4511fa4b30f73185597990ce563bbf3f96385292)
Link: https://github.com/openwrt/openwrt/pull/17097
Signed-off-by: Petr Štetiar <[email protected]>