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/am335x_shc_netboot_defconfig b/configs/am335x_shc_netboot_defconfig
index c63ffb7..504c932 100644
--- a/configs/am335x_shc_netboot_defconfig
+++ b/configs/am335x_shc_netboot_defconfig
@@ -18,6 +18,7 @@
 CONFIG_SHC_NETBOOT=y
 CONFIG_SERIES=y
 CONFIG_DISTRO_DEFAULTS=y
+CONFIG_TIMESTAMP=y
 CONFIG_SHOW_BOOT_PROGRESS=y
 CONFIG_AUTOBOOT_KEYED=y
 CONFIG_AUTOBOOT_PROMPT="Enter 'shc' to enter prompt (times out) %d \nEnter 'noautoboot' to enter prompt without timeout\n"