arch/x86/acpi: Allow "transparent" ACPI device names

Certain devices, such as the northbridge on AMD Opteron systems,
do not require a node in the ACPI device path.  Allow such devices
to be passed over by the ACPI path generator if the device-specific
ACPI name function returns a zero-length (non-NULL) string.

Change-Id: Iffffc9a30b395b0bd6d60e411439a437e89f554e
Signed-off-by: Timothy Pearson <tpearson@raptorengineering.com>
Reviewed-on: https://review.coreboot.org/19281
Tested-by: build bot (Jenkins)
Tested-by: Raptor Engineering Automated Test Stand <noreply@raptorengineeringinc.com>
Reviewed-by: Duncan Laurie <dlaurie@chromium.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/arch/x86/acpi_device.c

@@ -113,7 +113,9 @@ static size_t acpi_device_path_fill(struct device *dev, char *buf,
 
 	/* Fill in the path from the root device */
 	next += snprintf(buf + next, buf_len - next, "%s%s",
-			 dev->path.type == DEVICE_PATH_ROOT ? "" : ".", name);
+			 (dev->path.type == DEVICE_PATH_ROOT
+				|| (strlen(name) == 0)) ?
+					"" : ".", name);
 
 	return next;
 }