opkg: drop argument from check_signature in opkg.conf

opkg: drop argument from check_signature in opkg.conf

check_signature is a bool option and doesn't take any arguments. The
presence of the 1 falsely suggests setting it to 0 disables the check,
while the option actually needs to be removed or commented out to be
disabled. So remove the argument to make it more clear.

Fixes: beca028bd6bb ("build: add integration for managing opkg package feed keys")
Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>