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_evm_defconfig b/configs/k2l_evm_defconfig
index 6ef1de7..d800ea1 100644
--- a/configs/k2l_evm_defconfig
+++ b/configs/k2l_evm_defconfig
@@ -21,6 +21,7 @@
 CONFIG_SPL_SPI_FLASH_SUPPORT=y
 CONFIG_SPL_SPI=y
 CONFIG_DISTRO_DEFAULTS=y
+CONFIG_TIMESTAMP=y
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_BOOTCOMMAND="run init_${boot}; run get_mon_${boot} run_mon; run get_kern_${boot}; run init_fw_rd_${boot}; run get_fdt_${boot}; run run_kern"
 CONFIG_SYS_CONSOLE_INFO_QUIET=y