Convert CONFIG_SYS_MALLOC_LEN to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_MALLOC_LEN

Signed-off-by: Tom Rini <trini@konsulko.com>
Acked-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Rick Chen <rick@andestech.com>
diff --git a/configs/iot_devkit_defconfig b/configs/iot_devkit_defconfig
index a86a74c..e7d2cb3 100644
--- a/configs/iot_devkit_defconfig
+++ b/configs/iot_devkit_defconfig
@@ -6,6 +6,7 @@
 CONFIG_TARGET_IOT_DEVKIT=y
 CONFIG_SYS_TEXT_BASE=0x00000000
 CONFIG_ENV_SIZE=0x1000
+CONFIG_SYS_MALLOC_LEN=0x10000
 CONFIG_DEFAULT_DEVICE_TREE="iot_devkit"
 CONFIG_SYS_CLK_FREQ=16000000
 CONFIG_LOCALVERSION="-iotdk-1.0"