hostapd: recognize option "key" as alias for "auth_secret"

hostapd: recognize option "key" as alias for "auth_secret"

The hostapd configuration logic is supposed to accept "option key" as
legacy alias for "option auth_secret". This particular fallback option
failed to work though because "key" was not a registered configuration
variable.

Fix this issue by registering the "key" option as well, similar to the
existing "server" nad "port" options.

Ref: https://github.com/openwrt/openwrt/pull/3282
Suggested-by: Michael Jones <mike@meshplusplus.com>
Signed-off-by: Jo-Philipp Wich <jo@mein.io>