Dh/refactor plat header v3

This is a further rework of Dan's branch on top of Andrew's earlier pull request "Dh/refactor plat header v2" #106. The last four patches have undergone rework. An outstanding rename of plat_security.c to fvp_security.c was causing a build failure when RESET_TO_BL31 was set to 1. This is the only major functional fix.