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/ethernut5_defconfig b/configs/ethernut5_defconfig
index 971c8e0..91fa97d 100644
--- a/configs/ethernut5_defconfig
+++ b/configs/ethernut5_defconfig
@@ -9,6 +9,7 @@
 CONFIG_ENV_SIZE=0x21000
 CONFIG_ENV_OFFSET=0x3DE000
 CONFIG_ENV_SECT_SIZE=0x21000
+CONFIG_SYS_MALLOC_LEN=0x121000
 CONFIG_DM_GPIO=y
 CONFIG_DEFAULT_DEVICE_TREE="ethernut5"
 CONFIG_SYS_LOAD_ADDR=0x020000000