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-imx7d_defconfig b/configs/pico-imx7d_defconfig
index 2f07308..6473aaa 100644
--- a/configs/pico-imx7d_defconfig
+++ b/configs/pico-imx7d_defconfig
@@ -21,6 +21,8 @@
 CONFIG_IMX_RDC=y
 CONFIG_IMX_BOOTAUX=y
 CONFIG_DISTRO_DEFAULTS=y
+CONFIG_HAS_BOARD_SIZE_LIMIT=y
+CONFIG_BOARD_SIZE_LIMIT=715776
 CONFIG_BOOTCOMMAND="run findfdt; run finduuid; run distro_bootcmd"
 CONFIG_DEFAULT_FDT_FILE="ask"
 CONFIG_SPL_I2C=y