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/k2l_hs_evm_defconfig b/configs/k2l_hs_evm_defconfig
index 718ceff..1776217 100644
--- a/configs/k2l_hs_evm_defconfig
+++ b/configs/k2l_hs_evm_defconfig
@@ -13,6 +13,7 @@
 CONFIG_ENV_SIZE=0x40000
 CONFIG_ENV_OFFSET=0x100000
 CONFIG_DEFAULT_DEVICE_TREE="keystone-k2l-evm"
+CONFIG_TIMESTAMP=y
 CONFIG_FIT_IMAGE_POST_PROCESS=y
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_USE_BOOTCOMMAND=y