Convert CONFIG_ENV_SPI_BUS et al to Kconfig

This converts the following to Kconfig:
   CONFIG_ENV_SPI_BUS
   CONFIG_ENV_SPI_CS
   CONFIG_ENV_SPI_MAX_HZ
   CONFIG_ENV_SPI_MODE

As part of this, we use Kconfig to provide the defaults now that were
done in include/spi_flash.h.  We also in some cases change from using
CONFIG_ENV_SPI_FOO to CONFIG_SF_DEFAULT_FOO as those were the values in
use anyhow as ENV was not enabled.

Signed-off-by: Tom Rini <trini@konsulko.com>