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>
diff --git a/include/image.h b/include/image.h
index 4ad0e6b..d2325dd 100644
--- a/include/image.h
+++ b/include/image.h
@@ -333,6 +333,14 @@
 int genimg_get_comp_id(const char *name);
 void genimg_print_size(uint32_t size);
 
+#if defined(CONFIG_TIMESTAMP) || defined(CONFIG_CMD_DATE) || \
+	defined(USE_HOSTCC)
+#define IMAGE_ENABLE_TIMESTAMP 1
+#else
+#define IMAGE_ENABLE_TIMESTAMP 0
+#endif
+void genimg_print_time(time_t timestamp);
+
 #ifndef USE_HOSTCC
 /* Image format types, returned by _get_format() routine */
 #define IMAGE_FORMAT_INVALID	0x00