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>

README

@@ -1452,12 +1452,6 @@ Configuration Settings:
 - CONFIG_SYS_FLASH_BASE:
 		Physical start address of Flash memory.
 
-- CONFIG_SYS_MONITOR_LEN:
-		Size of memory reserved for monitor code, used to
-		determine _at_compile_time_ (!) if the environment is
-		embedded within the U-Boot image, or in a separate
-		flash sector.
-
 - CONFIG_SYS_MALLOC_LEN:
 		Size of DRAM reserved for malloc() use.