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/P2041RDB_NAND_defconfig b/configs/P2041RDB_NAND_defconfig
index 1ebc431..d979b14 100644
--- a/configs/P2041RDB_NAND_defconfig
+++ b/configs/P2041RDB_NAND_defconfig
@@ -2,6 +2,7 @@
 CONFIG_SYS_TEXT_BASE=0xFFF40000
 CONFIG_ENV_SIZE=0x20000
 CONFIG_ENV_OFFSET=0xE0000
+CONFIG_SYS_MALLOC_LEN=0x100000
 CONFIG_DEFAULT_DEVICE_TREE="p2041rdb"
 CONFIG_MPC85xx=y
 CONFIG_TARGET_P2041RDB=y