Convert CONFIG_SYS_MONITOR_LEN to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_MONITOR_LEN

To do this, we set a default of 0 for everyone because there are a
number of cases where we define CONFIG_SYS_MONITOR_LEN but the only
impact is that we set TOTAL_MALLOC_LEN to be CONFIG_SYS_MALLOC_LEN +
CONFIG_ENV_SIZE, so we must continue to allow all boards to set this
value. Update the SPL code to use 200 KB as the default raw U-Boot size
directly, if we don't have a real CONFIG_SYS_MONITOR_LEN value.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/pine64_plus_defconfig b/configs/pine64_plus_defconfig
index f42f4e5..0fd24b5 100644
--- a/configs/pine64_plus_defconfig
+++ b/configs/pine64_plus_defconfig
@@ -6,6 +6,7 @@
 CONFIG_RESERVE_ALLWINNER_BOOT0_HEADER=y
 CONFIG_PINE64_DT_SELECTION=y
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
+CONFIG_SYS_MONITOR_LEN=786432
 CONFIG_OF_LIST="sun50i-a64-pine64 sun50i-a64-pine64-plus"
 CONFIG_PHY_REALTEK=y
 CONFIG_SUN8I_EMAC=y