Build system: add 'make help' option

Add the 'help' target to the Makefile to present a brief guide to the
various build options available.

Signed-off-by: Ryan Harkin <ryan.harkin@linaro.org>
fvp: rename fvp_* files to plat_*

The FVP platform has a few filenames that begin with fvp_.  These are
renamed to plat_ to make it easier to use the FVP port as a template.

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