Convert CONFIG_BOARD_SIZE_LIMIT to Kconfig

This converts the following to Kconfig:
   CONFIG_BOARD_SIZE_LIMIT

To do this, introduce CONFIG_HAS_BOARD_SIZE_LIMIT.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/pico-imx6_defconfig b/configs/pico-imx6_defconfig
index 44d9295..0bd6613 100644
--- a/configs/pico-imx6_defconfig
+++ b/configs/pico-imx6_defconfig
@@ -18,6 +18,8 @@
 CONFIG_SPL=y
 CONFIG_SPL_LIBDISK_SUPPORT=y
 CONFIG_DISTRO_DEFAULTS=y
+CONFIG_HAS_BOARD_SIZE_LIMIT=y
+CONFIG_BOARD_SIZE_LIMIT=715776
 CONFIG_FIT=y
 CONFIG_SPL_FIT_PRINT=y
 CONFIG_SPL_LOAD_FIT=y