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/ap152_defconfig b/configs/ap152_defconfig
index e2f6f09..3aa0ea5 100644
--- a/configs/ap152_defconfig
+++ b/configs/ap152_defconfig
@@ -39,6 +39,7 @@
 CONFIG_MTDPARTS_DEFAULT="mtdparts=spi-flash.0:256k(u-boot),64k(u-boot-env),6336k(rootfs),1472k(uImage),64k(ART)"
 # CONFIG_ISO_PARTITION is not set
 CONFIG_ENV_IS_IN_SPI_FLASH=y
+CONFIG_ENV_SPI_MAX_HZ=25000000
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 # CONFIG_NET is not set
 CONFIG_MTD=y