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/tools-only_defconfig b/configs/tools-only_defconfig
index 1a0b41b..64a015b 100644
--- a/configs/tools-only_defconfig
+++ b/configs/tools-only_defconfig
@@ -1,5 +1,6 @@
 CONFIG_SYS_TEXT_BASE=0
 CONFIG_ENV_SIZE=0x2000
+CONFIG_SYS_MALLOC_LEN=0x2000000
 CONFIG_DEFAULT_DEVICE_TREE="sandbox"
 CONFIG_SYS_LOAD_ADDR=0x0
 CONFIG_ANDROID_BOOT_IMAGE=y