arm: boot: Add board specific setup code API

Add an API for boards to inject their own preboot software (or
firmware) sequence.

The software then returns to the bootloader via the link register. This
allows boards to do their own little bits of firmware setup without
needed to replace the bootloader completely (which is the requirement
for existing firmware support).

The blob is loaded by a callback if and only if doing a linux boot
(similar to the existing write_secondary support).

Rewrite the comment for the primary boot blob.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Message-id: 070295644c6ac84696d743913296e8cfefb48c15.1446182614.git.crosthwaite.peter@gmail.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>