common/console.c: ensure GD_FLG_SILENT is set or cleared

When CONFIG_SILENT_CONSOLE is defined and the default environment has
silent=1 it is not possible for a user to make the console un-silent if
the environment is not available when console_init_f() is called (for
example because the environment is in SPI).

Add a new helper function console_update_silent() and call it from both
console_init_f() and console_init_r().

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>