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/am335x_guardian_defconfig b/configs/am335x_guardian_defconfig
index c98bb61..da083c2 100644
--- a/configs/am335x_guardian_defconfig
+++ b/configs/am335x_guardian_defconfig
@@ -66,6 +66,7 @@
 CONFIG_SYS_REDUNDAND_ENVIRONMENT=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_SPL_ENV_IS_NOWHERE=y
+CONFIG_BOOTP_SEND_HOSTNAME=y
 CONFIG_SPL_DM=y
 CONFIG_BOOTCOUNT_LIMIT=y
 CONFIG_LED=y