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/M54418TWR_serial_mii_defconfig b/configs/M54418TWR_serial_mii_defconfig
index b2266a0..8b72bd3 100644
--- a/configs/M54418TWR_serial_mii_defconfig
+++ b/configs/M54418TWR_serial_mii_defconfig
@@ -20,6 +20,8 @@
 CONFIG_CMD_PING=y
 CONFIG_CMD_CACHE=y
 CONFIG_ENV_IS_IN_SPI_FLASH=y
+CONFIG_USE_ENV_SPI_CS=y
+CONFIG_ENV_SPI_CS=1
 CONFIG_SPI_FLASH=y
 CONFIG_SPI_FLASH_ATMEL=y
 CONFIG_MII=y