* Islamist bombings cause carnage in north, central Nigeria
* Militants still holding over 200 abducted school girls
* Boko Haram main security threat to Africa's top economy
By Lanre Ola and Imma Ande
MAIDUGURI/YOLA, Nigeria, June 1 (Reuters) - A bomb blast targeting a television viewing centre for football in northeast Nigeria killed at least 14 people and wounded 12 on Sunday, ...