Convert CONFIG_ENV_SPI_* 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

Most of time these value are not needed, CONFIG_SF_DEFAULT
with same value is used, so I introduced CONFIG_USE_ENV_SPI_*
to force the associated value for the environment.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
diff --git a/configs/snow_defconfig b/configs/snow_defconfig
index 6179a64..54bf740 100644
--- a/configs/snow_defconfig
+++ b/configs/snow_defconfig
@@ -34,6 +34,8 @@
 CONFIG_CMD_EXT4_WRITE=y
 CONFIG_DEFAULT_DEVICE_TREE="exynos5250-snow"
 CONFIG_ENV_IS_IN_SPI_FLASH=y
+CONFIG_USE_ENV_SPI_BUS=y
+CONFIG_ENV_SPI_BUS=1
 CONFIG_I2C_CROS_EC_LDO=y
 CONFIG_I2C_MUX=y
 CONFIG_I2C_ARB_GPIO_CHALLENGE=y