Move common source files to `common` directory

The top-level directory is getting increasingly cluttered. For clarity
let's move the common source files into their own directory. At the same
time let's clean up the way we generate object lists so that it's
consistent for arch/common objects, and doesn't require special casing
each optional object.

Note that we also need to create a common/ directory for out-of-tree
builds.

There should be no functional change as a result of this patch.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
8 files changed