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/am3517_evm_defconfig b/configs/am3517_evm_defconfig
index 833a55c..40e089e 100644
--- a/configs/am3517_evm_defconfig
+++ b/configs/am3517_evm_defconfig
@@ -50,6 +50,7 @@
 CONFIG_ENV_OVERWRITE=y
 # CONFIG_ENV_IS_IN_FAT is not set
 CONFIG_ENV_IS_IN_NAND=y
+CONFIG_BOOTP_SEND_HOSTNAME=y
 CONFIG_SPL_DM=y
 CONFIG_SPL_DM_SEQ_ALIAS=y
 CONFIG_SPL_OF_TRANSLATE=y