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_rtconly_defconfig b/configs/am43xx_evm_rtconly_defconfig
index a82c542..516c5c1 100644
--- a/configs/am43xx_evm_rtconly_defconfig
+++ b/configs/am43xx_evm_rtconly_defconfig
@@ -35,6 +35,7 @@
 CONFIG_ENV_OVERWRITE=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG=y
+CONFIG_BOOTP_SEND_HOSTNAME=y
 CONFIG_DM=y
 CONFIG_DFU_MMC=y
 CONFIG_DFU_RAM=y