Convert CONFIG_CMD_IOLOOP to Kconfig

This converts the following to Kconfig:
   CONFIG_CMD_IOLOOP

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/strider_cpu_defconfig b/configs/strider_cpu_defconfig
index 692e883..7d372ff 100644
--- a/configs/strider_cpu_defconfig
+++ b/configs/strider_cpu_defconfig
@@ -2,6 +2,7 @@
 CONFIG_IDENT_STRING=" strider cpu 0.01"
 CONFIG_MPC83xx=y
 CONFIG_TARGET_STRIDER=y
+CONFIG_CMD_IOLOOP=y
 CONFIG_FIT=y
 CONFIG_FIT_VERBOSE=y
 CONFIG_OF_BOARD_SETUP=y