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/q8_a23_tablet_800x480_defconfig b/configs/q8_a23_tablet_800x480_defconfig
index dda1a0c..dfa220b 100644
--- a/configs/q8_a23_tablet_800x480_defconfig
+++ b/configs/q8_a23_tablet_800x480_defconfig
@@ -16,6 +16,7 @@
 CONFIG_VIDEO_LCD_BL_EN="PH6"
 CONFIG_VIDEO_LCD_BL_PWM="PH0"
 # CONFIG_SYS_MALLOC_CLEAR_ON_INIT is not set
+CONFIG_SYS_MONITOR_LEN=786432
 CONFIG_AXP_DLDO1_VOLT=3300
 CONFIG_CONS_INDEX=5
 CONFIG_USB_MUSB_HOST=y