Fix fastboot flashall

The return value of load_buf was swapped so we were erroneously
failing.

Bug: 29278988
Change-Id: I777bfbc174f50ec0b71e973db36543eb37d160db
diff --git a/fastboot/fastboot.cpp b/fastboot/fastboot.cpp
index d35a874..8d16d20 100644
--- a/fastboot/fastboot.cpp
+++ b/fastboot/fastboot.cpp
@@ -1168,7 +1168,7 @@
         if (!slot) continue;
         fname = find_item_given_name(images[i].img_name, product);
         fastboot_buffer buf;
-        if (load_buf(transport, fname.c_str(), &buf)) {
+        if (!load_buf(transport, fname.c_str(), &buf)) {
             if (images[i].is_optional) continue;
             die("could not load %s\n", images[i].img_name);
         }