Fix test failure caused by bad handling of ramdisk

Commit e3a5bbce broke the FIT image tests by not loading a ramdisk even if
a load address is provided in the FIT. The rationale was that a load address
of 0 should be considered to mean 'do not load'.

Add a new load operation which supports this feature, so that the ramdisk
will be loaded if a non-zero load address is provided.

Signed-off-by: Simon Glass <sjg@chromium.org>

include/image.h

@@ -412,6 +412,7 @@ void genimg_print_time(time_t timestamp);
 enum fit_load_op {
 	FIT_LOAD_IGNORED,	/* Ignore load address */
 	FIT_LOAD_OPTIONAL,	/* Can be provided, but optional */
+	FIT_LOAD_OPTIONAL_NON_ZERO,	/* Optional, a value of 0 is ignored */
 	FIT_LOAD_REQUIRED,	/* Must be provided */
 };