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/stm32f429-discovery_defconfig b/configs/stm32f429-discovery_defconfig
index bb815e8..b6388cf 100644
--- a/configs/stm32f429-discovery_defconfig
+++ b/configs/stm32f429-discovery_defconfig
@@ -5,6 +5,7 @@
 CONFIG_NR_DRAM_BANKS=1
 CONFIG_ENV_SIZE=0x2000
 CONFIG_ENV_SECT_SIZE=0x20000
+CONFIG_SYS_MALLOC_LEN=0x0200000
 CONFIG_DEFAULT_DEVICE_TREE="stm32f429-disco"
 CONFIG_STM32F4=y
 CONFIG_TARGET_STM32F429_DISCOVERY=y