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/sandbox64_defconfig b/configs/sandbox64_defconfig
index a3f049e..6cb3aa4 100644
--- a/configs/sandbox64_defconfig
+++ b/configs/sandbox64_defconfig
@@ -49,6 +49,7 @@
 CONFIG_CMD_REMOTEPROC=y
 CONFIG_CMD_SPI=y
 CONFIG_CMD_USB=y
+CONFIG_BOOTP_DNS2=y
 CONFIG_CMD_TFTPPUT=y
 CONFIG_CMD_TFTPSRV=y
 CONFIG_CMD_RARP=y