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_defconfig b/configs/sandbox_defconfig
index 5b75693..20a81a0 100644
--- a/configs/sandbox_defconfig
+++ b/configs/sandbox_defconfig
@@ -56,6 +56,7 @@
 CONFIG_CMD_USB=y
 CONFIG_CMD_AXI=y
 CONFIG_CMD_AB_SELECT=y
+CONFIG_BOOTP_DNS2=y
 CONFIG_CMD_PCAP=y
 CONFIG_CMD_TFTPPUT=y
 CONFIG_CMD_TFTPSRV=y