Convert CONFIG_BOOTP_SEND_HOSTNAME to Kconfig

This converts the following to Kconfig:
   CONFIG_BOOTP_SEND_HOSTNAME

Signed-off-by: Adam Ford <aford173@gmail.com>
diff --git a/configs/da850evm_direct_nor_defconfig b/configs/da850evm_direct_nor_defconfig
index 0ae41dd..23725e0 100644
--- a/configs/da850evm_direct_nor_defconfig
+++ b/configs/da850evm_direct_nor_defconfig
@@ -42,6 +42,7 @@
 CONFIG_OF_CONTROL=y
 CONFIG_ENV_IS_IN_FLASH=y
 CONFIG_ENV_ADDR=0x60100000
+CONFIG_BOOTP_SEND_HOSTNAME=y
 CONFIG_DM=y
 CONFIG_BLK=y
 CONFIG_DA8XX_GPIO=y