By Amanda Ferguson BELFAST, Northern Ireland (Reuters) -British Prime Minister Boris Johnson said on Monday the government needed an “insurance” option to unilaterally scrap post-Brexit trade rules for Northern Ireland, raising the risk of a trade war with the European Union. Johnson says the EU must make concessions on the rules – known as the Northern Ireland protocol – to win over the province’s unionist community loyal to the United Kingdom, and has threatened action that the bloc says could start a trade war. The prime minister said plans to propose legislation to rewrite parts of the pro…