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/cubieboard7_defconfig b/configs/cubieboard7_defconfig
index da5806e..2937a54 100644
--- a/configs/cubieboard7_defconfig
+++ b/configs/cubieboard7_defconfig
@@ -1,5 +1,6 @@
 CONFIG_ARM=y
 CONFIG_ARCH_OWL=y
+CONFIG_SYS_MALLOC_LEN=0x2000000
 CONFIG_DEFAULT_DEVICE_TREE="s700-cubieboard7"
 CONFIG_MACH_S700=y
 CONFIG_IDENT_STRING="\ncubieboard7"