spl: Always define preloader_console_init

A large number of boards call preloader_console_init unconditionally.
Currently, they fail to build with CONFIG_SPL_SERIAL=n, because the
function is undefined in that case. To fix the build, always define
preloader_console_init, but make it no-op when CONFIG_SPL_SERIAL=n.

For the few boards that did check for CONFIG_SPL_SERIAL before calling
preloader_console_init, remove the checks, since the function can now
be called unconditionally.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>