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/sandbox_defconfig b/configs/sandbox_defconfig
index 0cfff0a..829056e 100644
--- a/configs/sandbox_defconfig
+++ b/configs/sandbox_defconfig
@@ -92,6 +92,7 @@
 CONFIG_OF_CONTROL=y
 CONFIG_OF_LIVE=y
 CONFIG_OF_HOSTFILE=y
+CONFIG_BOOTP_SEND_HOSTNAME=y
 CONFIG_NETCONSOLE=y
 CONFIG_IP_DEFRAG=y
 CONFIG_REGMAP=y