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/adp-ae3xx_defconfig b/configs/adp-ae3xx_defconfig
index 22e18c8..1e48a42 100644
--- a/configs/adp-ae3xx_defconfig
+++ b/configs/adp-ae3xx_defconfig
@@ -24,6 +24,7 @@
 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_DM=y
 CONFIG_CLK=y