Convert CONFIG_ENV_IS_IN_EEPROM to Kconfig

This converts the following to Kconfig:
   CONFIG_ENV_IS_IN_EEPROM

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/imx31_phycore_defconfig b/configs/imx31_phycore_defconfig
index 224bedb..a710e06 100644
--- a/configs/imx31_phycore_defconfig
+++ b/configs/imx31_phycore_defconfig
@@ -1,5 +1,6 @@
 CONFIG_ARM=y
 CONFIG_TARGET_IMX31_PHYCORE=y
+CONFIG_ENV_IS_IN_EEPROM=y
 CONFIG_BOOTDELAY=3
 CONFIG_SYS_PROMPT="uboot> "
 CONFIG_CMD_EEPROM=y