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/sandbox_spl_defconfig b/configs/sandbox_spl_defconfig
index fc8b26e..d499ac1 100644
--- a/configs/sandbox_spl_defconfig
+++ b/configs/sandbox_spl_defconfig
@@ -51,6 +51,7 @@
 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