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_shc_sdboot_defconfig b/configs/am335x_shc_sdboot_defconfig
index 9e60b62..a28ed1e 100644
--- a/configs/am335x_shc_sdboot_defconfig
+++ b/configs/am335x_shc_sdboot_defconfig
@@ -50,6 +50,7 @@
 CONFIG_SYS_REDUNDAND_ENVIRONMENT=y
 CONFIG_SYS_RELOC_GD_ENV_ADDR=y
 CONFIG_ENV_VARS_UBOOT_RUNTIME_CONFIG=y
+CONFIG_BOOTP_SEND_HOSTNAME=y
 CONFIG_DM_MMC=y
 CONFIG_MMC_OMAP_HS=y
 CONFIG_PHY_ADDR_ENABLE=y