Convert CONFIG_TIMESTAMP to Kconfig

This converts the following to Kconfig:
   CONFIG_TIMESTAMP

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/chiliboard_defconfig b/configs/chiliboard_defconfig
index 023cd19..a50e732 100644
--- a/configs/chiliboard_defconfig
+++ b/configs/chiliboard_defconfig
@@ -16,6 +16,7 @@
 CONFIG_SPL_FS_FAT=y
 CONFIG_SPL_LIBDISK_SUPPORT=y
 CONFIG_DISTRO_DEFAULTS=y
+CONFIG_TIMESTAMP=y
 CONFIG_BOOTDELAY=1
 CONFIG_BOOTCOMMAND="run mmcboot; run nandboot; run netboot"
 CONFIG_DEFAULT_FDT_FILE="am335x-chiliboard.dtb"