firewall/firewall4: provide uci-firewall

firewall/firewall4: provide uci-firewall

Provide uci-firewall via PROVIDES in both firewall and firewall4. This
will allow us to change the dependency of luci-app-firewall to
uci-firewall, making it possible to use it with either implementation.

Move CONFLICTS from firewall4 to firewall, to solve this recursive
dependency problem:

tmp/.config-package.in:307:error: recursive dependency detected!
tmp/.config-package.in:307:     symbol PACKAGE_firewall is selected by PACKAGE_firewall4
tmp/.config-package.in:328:     symbol PACKAGE_firewall4 depends on PACKAGE_firewall

Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>
Reviewed-by: Jo-Philipp Wich <jo@mein.io>