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/dreamplug_defconfig b/configs/dreamplug_defconfig
index d3263cf..8202848 100644
--- a/configs/dreamplug_defconfig
+++ b/configs/dreamplug_defconfig
@@ -23,6 +23,8 @@
 CONFIG_OF_CONTROL=y
 CONFIG_DEFAULT_DEVICE_TREE="kirkwood-dreamplug"
 CONFIG_ENV_IS_IN_SPI_FLASH=y
+CONFIG_USE_ENV_SPI_MAX_HZ=y
+CONFIG_ENV_SPI_MAX_HZ=50000000
 CONFIG_DM=y
 CONFIG_MVSATA_IDE=y
 # CONFIG_MMC is not set