drivers/console: Link console framework code by default

This patch makes the build system link the console framework code by
default, like it already does with other common libraries (e.g. cache
helpers). This should not make a difference in practice since TF is
linked with --gc-sections, so the linker will garbage collect all
functions and data that are not referenced by any other code. Thus, if a
platform doesn't want to include console code for size reasons and
doesn't make any references to console functions, the code will not be
included in the final binary.

To avoid compatibility issues with older platform ports, only make this
change for the MULTI_CONSOLE_API.

Change-Id: I153a9dbe680d57aadb860d1c829759ba701130d3
Signed-off-by: Julius Werner <jwerner@chromium.org>