console: Print out complete stdio device list

At present if both CONSOLE_MUX and SYS_CONSOLE_IS_IN_ENV are on,
during boot, the printed out stdio devices are incomplete, e.g.:
with "stdout=serial,vidconsole", only "vidconsole" is printed.

For such case, we can print out the stdio device name from the
environment variables.

Signed-off-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>