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/at91sam9rlek_dataflash_defconfig b/configs/at91sam9rlek_dataflash_defconfig
index 61cd9b9..444e43e 100644
--- a/configs/at91sam9rlek_dataflash_defconfig
+++ b/configs/at91sam9rlek_dataflash_defconfig
@@ -9,6 +9,7 @@
 CONFIG_ENV_SIZE=0x4200
 CONFIG_ENV_OFFSET=0x4200
 CONFIG_ENV_SECT_SIZE=0x210
+CONFIG_SYS_MALLOC_LEN=0x2d000
 CONFIG_DM_GPIO=y
 CONFIG_DEFAULT_DEVICE_TREE="at91sam9rlek"
 CONFIG_DEBUG_UART_BOARD_INIT=y