arm: config: fix default console only to specify the device

Since CONFIG_DEFAULT_CONSOLE is already started with "console=",
the console argument in CONFIG_EXTRA_ENV_SETTINGS is expanded to
"console=console=ttySAC1,115200n8" and this causes the wrong
console device.

  #define CONFIG_DEFAULT_CONSOLE         "console=ttySAC1,115200n8\0"
  ...
  #define CONFIG_EXTRA_ENV_SETTINGS \
		"console=" CONFIG_DEFAULT_CONSOLE

Signed-off-by: Dongjin Kim <tobetter@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>