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/boston32r6_defconfig b/configs/boston32r6_defconfig
index de8cc25..1c5a0a9 100644
--- a/configs/boston32r6_defconfig
+++ b/configs/boston32r6_defconfig
@@ -4,6 +4,7 @@
 CONFIG_SYS_MEMTEST_END=0x90000000
 CONFIG_ENV_SIZE=0x20000
 CONFIG_ENV_SECT_SIZE=0x20000
+CONFIG_SYS_MALLOC_LEN=0x40000
 CONFIG_DEFAULT_DEVICE_TREE="img,boston"
 CONFIG_TARGET_BOSTON=y
 CONFIG_CPU_MIPS32_R6=y