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/am335x_hs_evm_defconfig b/configs/am335x_hs_evm_defconfig
index f97ecac..8a7a9bb 100644
--- a/configs/am335x_hs_evm_defconfig
+++ b/configs/am335x_hs_evm_defconfig
@@ -39,6 +39,7 @@
 CONFIG_ENV_OVERWRITE=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG=y
+CONFIG_BOOTP_SEND_HOSTNAME=y
 CONFIG_BOOTCOUNT_LIMIT=y
 CONFIG_CLK=y
 CONFIG_CLK_CDCE9XX=y