RFC: bootm: Add silent_linux environment variable

At present the console for linux is silent if the U-Boot console is silent,
unless CONFIG_SILENT_U_BOOT_ONLY is set. I wonder if a better way would be
to have an environment variable to control this? Then we can control the
verbosity from scripts, and set the variable to 'no' for those boards that
want Linux to boot with console output.

Signed-off-by: Simon Glass <sjg@chromium.org>