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_rv32_spl_xip_defconfig b/configs/ae350_rv32_spl_xip_defconfig
index 1eb6c06..3448c79 100644
--- a/configs/ae350_rv32_spl_xip_defconfig
+++ b/configs/ae350_rv32_spl_xip_defconfig
@@ -21,6 +21,7 @@
 CONFIG_OF_BOARD=y
 CONFIG_ENV_OVERWRITE=y
 CONFIG_ENV_IS_IN_SPI_FLASH=y
+CONFIG_BOOTP_SEND_HOSTNAME=y
 CONFIG_NET_RANDOM_ETHADDR=y
 CONFIG_MMC=y
 CONFIG_FTSDC010=y