sdk: add OpenWrt branding to menuconfig & .config

Set the mainmenu symbol in SDK Config.in to "OpenWrt Configuration", the
same as the main OpenWrt Config.in.  This string is is used as the name
of the top menu in menuconfig, and at the top of the .config file.  If
unset, current kconfig will use "Linux Kernel Configuration".

Signed-off-by: Eneas U de Queiroz <cotequeiroz@gmail.com>

target/sdk/files/Config.in

@@ -1,3 +1,5 @@
+mainmenu "OpenWrt Configuration"
+
 menu "Global build settings"
 
 	config ALL_NONSHARED