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_evm_defconfig b/configs/am335x_evm_defconfig
index 57bd6be..0c05c32 100644
--- a/configs/am335x_evm_defconfig
+++ b/configs/am335x_evm_defconfig
@@ -42,6 +42,7 @@
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG=y
 CONFIG_SPL_ENV_IS_NOWHERE=y
+CONFIG_BOOTP_SEND_HOSTNAME=y
 CONFIG_BOOTCOUNT_LIMIT=y
 CONFIG_CLK=y
 CONFIG_CLK_CDCE9XX=y