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/r2dplus_defconfig b/configs/r2dplus_defconfig
index 6045e06..cee9a1f 100644
--- a/configs/r2dplus_defconfig
+++ b/configs/r2dplus_defconfig
@@ -9,6 +9,7 @@
 CONFIG_SYS_LOAD_ADDR=0x8e000000
 CONFIG_ENV_ADDR=0xA0040000
 CONFIG_TARGET_R2DPLUS=y
+CONFIG_SYS_MONITOR_LEN=262144
 CONFIG_SYS_MONITOR_BASE=0xA0000000
 CONFIG_BOOTDELAY=-1
 CONFIG_USE_BOOTARGS=y