console: Squelch pre-console output in console functions

There are some locations in the code which anticipate printf() being called
before the console is ready by squelching printf() on gd->have_console.
Move this squelching into printf(), vprintf(), puts() and putc(). Also
make tstc() and getc() return 0 if console is not yet initialised

Signed-off-by: Graeme Russ <graeme.russ@gmail.com>
Tested-by: Simon Glass <sjg@chromium.org>