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/am3517_evm_defconfig b/configs/am3517_evm_defconfig
index 4d3d02d..45332e2 100644
--- a/configs/am3517_evm_defconfig
+++ b/configs/am3517_evm_defconfig
@@ -17,6 +17,7 @@
 CONFIG_DISTRO_DEFAULTS=y
 CONFIG_HAS_CUSTOM_SYS_INIT_SP_ADDR=y
 CONFIG_CUSTOM_SYS_INIT_SP_ADDR=0x4020ff00
+CONFIG_SYS_MONITOR_LEN=262144
 CONFIG_BOOTDELAY=10
 CONFIG_BOOTCOMMAND="mmc dev ${mmcdev}; if mmc rescan; then echo SD/MMC found on device $mmcdev; if run loadbootenv; then run importbootenv; fi; echo Checking if uenvcmd is set ...; if test -n $uenvcmd; then echo Running uenvcmd ...; run uenvcmd; fi; echo Running default loadimage ...; setenv bootfile zImage; if run loadimage; then run loadfdt; run mmcboot; fi; else run nandboot; fi"
 CONFIG_SPL_MAX_SIZE=0xec00