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/smdk5250_defconfig b/configs/smdk5250_defconfig
index d24e9e2..6741387 100644
--- a/configs/smdk5250_defconfig
+++ b/configs/smdk5250_defconfig
@@ -30,6 +30,8 @@
 CONFIG_CMD_EXT4_WRITE=y
 CONFIG_DEFAULT_DEVICE_TREE="exynos5250-smdk5250"
 CONFIG_ENV_IS_IN_SPI_FLASH=y
+CONFIG_USE_ENV_SPI_BUS=y
+CONFIG_ENV_SPI_BUS=1
 CONFIG_MMC_DW=y
 CONFIG_MMC_SDHCI=y
 CONFIG_MMC_SDHCI_S5P=y