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/am335x_sl50_defconfig b/configs/am335x_sl50_defconfig
index 0aeaad5..5ba7b91 100644
--- a/configs/am335x_sl50_defconfig
+++ b/configs/am335x_sl50_defconfig
@@ -38,6 +38,7 @@
 CONFIG_CMD_MMC=y
 CONFIG_CMD_SPI=y
 # CONFIG_CMD_SETEXPR is not set
+CONFIG_BOOTP_DNS2=y
 CONFIG_CMD_EXT4_WRITE=y
 CONFIG_OF_CONTROL=y
 CONFIG_DEFAULT_DEVICE_TREE="am335x-sl50"