TBB: rework cert_create tool to follow a data driven approach ...

This patch reworks the certificate generation tool to follow a data
driven approach. The user may specify at build time the certificates,
keys and extensions defined in the CoT, register them using the
appropiate macros and the tool will take care of creating the
certificates corresponding to the CoT specified.

Change-Id: I29950b39343c3e1b71718fce0e77dcf2a9a0be2f

TBB: add ECDSA support to the certificate generation tool ...

This patch extends the 'cert_create' tool to support ECDSA keys
to sign the certificates. The '--key-alg' command line option
can be used to specify the key algorithm when invoking the tool.
Available options are:

    * 'rsa': create RSA-2048 keys (default option)
    * 'ecdsa': create ECDSA-SECP256R1 keys

The TF Makefile has been updated to allow the platform to specify
the key algorithm by declaring the 'KEY_ALG' variable in the
platform makefile.

The behaviour regarding key management has changed. After applying
this patch, the tool will try first to open the keys from disk. If
one key does not exist or no key is specified, and the command line
option to create keys has been specified, new keys will be created.
Otherwise an error will be generated and the tool will exit. This
way, the user may specify certain keys while the tool will create
the remaining ones. This feature is useful for testing purposes
and CI infrastructures.

The OpenSSL directory may be specified using the build option
'OPENSSL_DIR' when building the certificate generation tool.
Default is '/usr'.

Change-Id: I98bcc2bfab28dd7179f17f1177ea7a65698df4e7

Allow deeper platform port directory structure ...

Update the top level makefile to allow platform ports to exist in
subdirectories at any level instead of one level under `plat/`. The
makefile recursively searches for all files called `platform.mk` in
all subdirectories of `plat/`. The directory containing
`platform.mk` is the platform name. Platform names must be unique
across the codebase.

Replace usage of HELP_PLATFORMS in the Makefile with PLATFORMS since
these are both used to report the same information back to the user.

Update the TSP and cert_create tool makefiles in a similar way
to support a deeper platform port directory structure.

Also add PLAT_<plat_name> as a define passed through the top level
makefile to the source files, to allow build time variation in common
platform code.

Change-Id: I213420164808c5ddb99a26144e8e3f141a7417b7

TBB: add tool to generate certificates ...

This patch adds a tool that generates all the necessary elements
to establish the chain of trust (CoT) between the images.

The tool reads the binary images and signing keys and outputs the
corresponding certificates that will be used by the target at run
time to verify the authenticity of the images.

Note: the platform port must provide the file platform_oid.h. This
file will define the OIDs of the x509 extensions that will be added
to the certificates in order to establish the CoT.

Change-Id: I2734d6808b964a2107ab3a4805110698066a04be