Convert CONFIG_BOOTP_DNS2 to Kconfig

This converts the following to Kconfig:
   CONFIG_BOOTP_DNS2
   CONFIG_BOOTP_PXE_CLIENTARCH

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/thuban_defconfig b/configs/thuban_defconfig
index d81981a..59b99cd 100644
--- a/configs/thuban_defconfig
+++ b/configs/thuban_defconfig
@@ -47,6 +47,7 @@
 CONFIG_CMD_USB=y
 # CONFIG_CMD_SETEXPR is not set
 CONFIG_CMD_DHCP=y
+CONFIG_BOOTP_DNS2=y
 CONFIG_CMD_MII=y
 CONFIG_CMD_PING=y
 CONFIG_CMD_CACHE=y