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>
diff --git a/configs/p3450-0000_defconfig b/configs/p3450-0000_defconfig
index 85402e9..c1ce890 100644
--- a/configs/p3450-0000_defconfig
+++ b/configs/p3450-0000_defconfig
@@ -32,6 +32,7 @@
 CONFIG_ENV_OVERWRITE=y
 # CONFIG_ENV_IS_IN_MMC is not set
 CONFIG_ENV_IS_IN_SPI_FLASH=y
+CONFIG_ENV_SPI_MAX_HZ=48000000
 CONFIG_DFU_MMC=y
 CONFIG_DFU_RAM=y
 CONFIG_DFU_SF=y