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/ae350_rv64_xip_defconfig b/configs/ae350_rv64_xip_defconfig
index 2b0c29d..60a6901 100644
--- a/configs/ae350_rv64_xip_defconfig
+++ b/configs/ae350_rv64_xip_defconfig
@@ -20,6 +20,7 @@
 CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
+CONFIG_BOOTP_SEND_HOSTNAME=y
 CONFIG_NET_RANDOM_ETHADDR=y
 CONFIG_MMC=y
 CONFIG_FTSDC010=y