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/tools-only_defconfig b/configs/tools-only_defconfig
index bfa3138..a853abf 100644
--- a/configs/tools-only_defconfig
+++ b/configs/tools-only_defconfig
@@ -12,6 +12,7 @@
 CONFIG_OF_CONTROL=y
 CONFIG_OF_HOSTFILE=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
+CONFIG_BOOTP_SEND_HOSTNAME=y
 CONFIG_IP_DEFRAG=y
 # CONFIG_ACPIGEN is not set
 CONFIG_AXI=y