Fix build error when `BL32` is not defined ...

If an SPD wants to use a prebuilt binary as BL32 image (for example,
the OPTEE Dispatcher), it must point the `BL32` variable to the
image file. This dependency should apply only to the `fip` target.
However, it also applies to the `all` target at the moment. If the
user tries to build all individual TF images using `make all`
without setting BL32, the build fails. The following command will
throw the error:

    make CROSS_COMPILE=aarch64-linux-gnu- SPD=opteed all
    ...
    ...
    aarch64-linux-gnu-gcc: fatal error: no input files
    compilation terminated.
    make: *** [build/fvp/release/bl32/bl32.ld] Error 1

The reason is that the build system checks if BL32 is defined, and
if it is not, it will try to build BL32 from source. If the SPD
makefile does not provide support for that (as is the case of the
OPTEE Dispatcher, since OPTEE is provided as an external binary),
the build will fail.

This patch fixes the issue by checking if `BL32_SOURCES` has been
defined by the SPD before attempting to build BL32 from source.
If neither `BL32` nor `BL32_SOURCES` is defined when building the
FIP, a warning message will be printed and the process aborted.

Fixes ARM-software/tf-issues#333

Change-Id: I5e801ad333103ed9b042e5c4757424c8df2ff6e4

Rework Makefile ...

This patch is a complete rework of the main Makefile. Functionality
remains the same but the code has been reorganized in sections in
order to improve readability and facilitate adding future extensions.

A new file 'build_macros.mk' has been created and will contain common
definitions (variables, macros, etc) that may be used from the main
Makefile and other platform specific makefiles.

A new macro 'FIP_ADD_IMG' has been introduced and it will allow the
platform to specify binary images and the necessary checks for a
successful build. Platforms that require a BL30 image no longer need
to specify the NEED_BL30 option. The main Makefile is now completely
unaware of additional images not built as part of Trusted Firmware,
like BL30. It is the platform responsibility to specify images using
the macro 'FIP_ADD_IMG'. Juno uses this macro to include the BL30
image in the build.

BL33 image is specified in the main Makefile to preserve backward
compatibility with the NEED_BL33 option. Otherwise, platform ports
that rely on the definition of NEED_BL33 might break.

All Trusted Board Boot related definitions have been moved to a
separate file 'tbbr_tools.mk'. The main Makefile will include this
file unless the platform indicates otherwise by setting the variable
'INCLUDE_TBBR_MK := 0' in the corresponding platform.mk file. This
will keep backward compatibility but ideally each platform should
include the corresponding TBB .mk file in platform.mk.

Change-Id: I35e7bc9930d38132412e950e20aa2a01e2b26801