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>

package/network/services/hostapd/Makefile

@@ -7,7 +7,7 @@
 include $(TOPDIR)/rules.mk
 
 PKG_NAME:=hostapd
-PKG_RELEASE:=3
+PKG_RELEASE:=4
 
 PKG_SOURCE_URL:=http://w1.fi/hostap.git
 PKG_SOURCE_PROTO:=git