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/da850evm_defconfig b/configs/da850evm_defconfig
index 0df267d..83a9443 100644
--- a/configs/da850evm_defconfig
+++ b/configs/da850evm_defconfig
@@ -54,6 +54,7 @@
 CONFIG_SPL_OF_CONTROL=y
 CONFIG_ENV_IS_IN_SPI_FLASH=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
+CONFIG_BOOTP_SEND_HOSTNAME=y
 CONFIG_DM=y
 CONFIG_SPL_DM=y
 CONFIG_SPL_DM_SEQ_ALIAS=y