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/bananapi_m2_zero_defconfig b/configs/bananapi_m2_zero_defconfig
index ac3f8f5..b351d50 100644
--- a/configs/bananapi_m2_zero_defconfig
+++ b/configs/bananapi_m2_zero_defconfig
@@ -6,3 +6,4 @@
 CONFIG_DRAM_CLK=408
 CONFIG_MMC0_CD_PIN=""
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
+CONFIG_SYS_MONITOR_LEN=786432