Separate out common console functionality ...

Separate out the common console functionality in
`drivers/arm/pl011/pl011_console.S` into a new source file
`drivers/console/console.S`. The former includes the latter to
provide backwards compatibility for platform make files.

Also add a skeleton console implementation for platforms that do not
want to use PL011.

Change-Id: I1ff963b2b54a872fbcf1eb0700797b9e9afa2538