image-host: Fix error value paths and emit error messages to stderr.

A recent refactoring in image-host.c messed up the return values of
the function that reads the encryptiong keys.  This patch fixes this
and also makes sure that error output goes to stderr instead of to
stdout.

Signed-off-by: Hugo Cornelis <hugo.cornelis@essensium.com>
diff --git a/tools/image-host.c b/tools/image-host.c
index b2a0f2e..7bfc0cb 100644
--- a/tools/image-host.c
+++ b/tools/image-host.c
@@ -346,17 +346,17 @@
 				      unsigned char *key_iv_data, int expected_size)
 {
 	char filename[PATH_MAX];
-	int ret = -1;
+	int ret;
 
 	ret = snprintf(filename, sizeof(filename), "%s/%s%s",
 		       keydir, key_iv_name, ".bin");
 	if (ret >= sizeof(filename)) {
-		printf("Can't format the key or IV filename when setting up the cipher: insufficient buffer space\n");
-		ret = -1;
+		fprintf(stderr, "Can't format the key or IV filename when setting up the cipher: insufficient buffer space\n");
+		return -1;
 	}
 	if (ret < 0) {
-		printf("Can't format the key or IV filename when setting up the cipher: snprintf error\n");
-		ret = -1;
+		fprintf(stderr, "Can't format the key or IV filename when setting up the cipher: snprintf error\n");
+		return -1;
 	}
 
 	ret = fit_image_read_data(filename, key_iv_data, expected_size);