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/lx2160ardb_tfa_stmm_defconfig b/configs/lx2160ardb_tfa_stmm_defconfig
index d10431e..42b6421 100644
--- a/configs/lx2160ardb_tfa_stmm_defconfig
+++ b/configs/lx2160ardb_tfa_stmm_defconfig
@@ -27,6 +27,7 @@
 CONFIG_SYS_FSL_NUM_CC_PLLS=4
 CONFIG_FSL_QIXIS=y
 CONFIG_REMAKE_ELF=y
+CONFIG_SYS_MONITOR_LEN=958464
 CONFIG_MP=y
 CONFIG_FIT_VERBOSE=y
 CONFIG_OF_BOARD_SETUP=y