Fix semihosting with latest toolchain ...

Fixes issues #10:

https://github.com/ARM-software/tf-issues/issues/10

This patch changes all/most variables of type int to be size_t or long
to fix the sizing and alignment problems found when building with the
newer toolchains such as Linaro GCC 13.12 or later.

Change-Id: Idc9d48eb2ff9b8c5bbd5b227e6907263d1ea188b
Signed-off-by: Ryan Harkin <ryan.harkin@linaro.org>

Implement load_image in terms of IO abstraction ...

The modified implementation uses the IO abstraction rather than
making direct semi-hosting calls.  The semi-hosting driver is now
registered for the FVP platform during initialisation of each boot
stage where it is used.  Additionally, the FVP platform includes a
straightforward implementation of 'plat_get_image_source' which
provides a generic means for the 'load_image' function to determine
how to access the image data.

Change-Id: Ia34457b471dbee990c7b3c79de7aee4ceea51aa6