Convert CONFIG_SYS_FAULT_ECHO_LINK_DOWN to Kconfig

This converts the following to Kconfig:
   CONFIG_SYS_FAULT_ECHO_LINK_DOWN

Cc: Ramon Fried <rfried.dev@gmail.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/kmnusa_defconfig b/configs/kmnusa_defconfig
index f7f9a8b..690cb3c 100644
--- a/configs/kmnusa_defconfig
+++ b/configs/kmnusa_defconfig
@@ -53,6 +53,7 @@
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_VERSION_VARIABLE=y
 CONFIG_NETCONSOLE=y
+CONFIG_SYS_FAULT_ECHO_LINK_DOWN=y
 CONFIG_BOOTCOUNT_LIMIT=y
 CONFIG_BOOTCOUNT_RAM=y
 CONFIG_KIRKWOOD_GPIO=y