image: Move timestamp #ifdefs to header file

Rather than repeat the line
 #if defined(CONFIG_TIMESTAMP) || defined(CONFIG_CMD_DATE) || \
	defined(USE_HOSTCC)

everywhere, put this in a header file and #define IMAGE_ENABLE_TIMESTAMP
to either 1 or 0. Then we can use a plain if() in most code and avoid
the #ifdefs.

The compiler's dead code elimination ensures that the result is the same.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Marek Vasut <marex@denx.de>