configuregateway: Only match "function" in subshell

When running OpenWrt master, "type <functionname>" only returns
"<functionname> is a function". Thus, to make the implementation
future-proof and since it is also working with the current state,
change the grep in execute_subshell to match "function".

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Reviewed-by: Fabian Bläse <fabian@blaese.de>
Reviewed-by: Robert Langhammer <rlanghammer@web.de>