arm: boot: Add secure_board_setup flag

Add a flag that when set, will cause the primary CPU to start in secure
mode, even if the overall boot is non-secure. This is useful for when
there is a board-setup blob that needs to run from secure mode, but
device and secondary CPU init should still be done as-normal for a non-
secure boot.

Signed-off-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Message-id: d1170774d5446d715fced7739edfc61a5be931f9.1447007690.git.crosthwaite.peter@gmail.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>