Convert CONFIG_SPL_BSS_MAX_SIZE et al to Kconfig

This converts the following to Kconfig:
   CONFIG_SPL_BSS_MAX_SIZE
   CONFIG_SPL_MAX_FOOTPRINT

Note that the da850evm platforms were violating the "only use one" rule
here, and so now hard-code their BSS limit.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/imx8mn_beacon_2g_defconfig b/configs/imx8mn_beacon_2g_defconfig
index aec7ddd..905d3e5 100644
--- a/configs/imx8mn_beacon_2g_defconfig
+++ b/configs/imx8mn_beacon_2g_defconfig
@@ -33,6 +33,7 @@
 CONFIG_DEFAULT_FDT_FILE="imx8mn-beacon-kit.dtb"
 CONFIG_ARCH_MISC_INIT=y
 CONFIG_SPL_MAX_SIZE=0x25000
+CONFIG_SPL_BSS_MAX_SIZE=0x2000
 CONFIG_SPL_BOARD_INIT=y
 CONFIG_SPL_BOOTROM_SUPPORT=y
 CONFIG_SYS_MMCSD_RAW_MODE_U_BOOT_USE_SECTOR=y