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/am43xx_evm_usbhost_boot_defconfig b/configs/am43xx_evm_usbhost_boot_defconfig
index bcc6324..967c88c 100644
--- a/configs/am43xx_evm_usbhost_boot_defconfig
+++ b/configs/am43xx_evm_usbhost_boot_defconfig
@@ -49,6 +49,7 @@
 CONFIG_ENV_FAT_DEVICE_AND_PART="0:1"
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG=y
+CONFIG_BOOTP_SEND_HOSTNAME=y
 CONFIG_DM=y
 CONFIG_REGMAP=y
 CONFIG_SPL_REGMAP=y