Factor out cross-BL API into export headers suitable for 3rd party code ...

This patch adds a new include/export/ directory meant for inclusion in
third-party code. This is useful for cases where third-party code needs
to interact with TF-A interfaces and data structures (such as a custom
BL2-implementation like coreboot handing off to BL31). Directly
including headers from the TF-A repository avoids having to duplicate
all these definitions (and risk them going stale), but with the current
header structure this is not possible because handoff API definitions
are too deeply intertwined with other TF code/headers and chain-include
other headers that will not be available in the other environment.

The new approach aims to solve this by separating only the parts that
are really needed into these special headers that are self-contained and
will not chain-include other (non-export) headers. TF-A code should
never include them directly but should instead always include the
respective wrapper header, which will include the required prerequisites
(like <stdint.h>) before including the export header. Third-party code
can include the export headers via its own wrappers that make sure the
necessary definitions are available in whatever way that environment can
provide them.

Change-Id: Ifd769320ba51371439a8e5dd5b79c2516c3b43ab
Signed-off-by: Julius Werner <jwerner@chromium.org>

Introduce lightweight BL platform parameter library ...

This patch adds some common helper code to support a lightweight
platform parameter passing framework between BLs that has already been
used on Rockchip platforms but is more widely useful to others as well.
It can be used as an implementation for the SoC firmware configuration
file mentioned in the docs, and is primarily intended for platforms
that only require a handful of values to be passed and want to get by
without a libfdt dependency. Parameters are stored in a linked list and
the parameter space is split in generic and vendor-specific parameter
types. Generic types will be handled by this code whereas
vendor-specific types have to be handled by a vendor-specific handler
function that gets passed in.

Change-Id: If3413d44e86b99d417294ce8d33eb2fc77a6183f
Signed-off-by: Julius Werner <jwerner@chromium.org>