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