This patch updates the WaRP7 build descriptions for booting WaRP7 in
Trusted Board Boot mode. TBB is the only mode we really intend to support
for this board so rather than maintain documentation for the old way of
doing it, this patch updates the description for TBB mode only.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>