Convert CONFIG_CMD_ENV_CALLBACK to Kconfig

This converts the following to Kconfig:
   CONFIG_CMD_ENV_CALLBACK

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/sandbox_spl_defconfig b/configs/sandbox_spl_defconfig
index cdeb30c..b623e29 100644
--- a/configs/sandbox_spl_defconfig
+++ b/configs/sandbox_spl_defconfig
@@ -30,6 +30,7 @@
 # CONFIG_CMD_IMLS is not set
 CONFIG_CMD_ASKENV=y
 CONFIG_CMD_GREPENV=y
+CONFIG_CMD_ENV_CALLBACK=y
 CONFIG_CMD_MD5SUM=y
 CONFIG_LOOPW=y
 CONFIG_CMD_MEMTEST=y