Fix ParseInt/ParseUint to handle explicit "0x" hex.

Also improve fastboot error reporting around max-download-size.

Change-Id: Ic3aec9460de01e5264a2803a0a6be3706d73026b
diff --git a/fastboot/fastboot.cpp b/fastboot/fastboot.cpp
index 7bf4b31..6b845a0 100644
--- a/fastboot/fastboot.cpp
+++ b/fastboot/fastboot.cpp
@@ -571,11 +571,13 @@
 static int64_t get_target_sparse_limit(usb_handle* usb) {
     std::string max_download_size;
     if (!fb_getvar(usb, "max-download-size", &max_download_size)) {
+        fprintf(stderr, "target didn't report max-download-size\n");
         return 0;
     }
 
     uint64_t limit;
     if (!android::base::ParseUint(max_download_size.c_str(), &limit)) {
+        fprintf(stderr, "couldn't parse max-download-size '%s'\n", max_download_size.c_str());
         return 0;
     }
     if (limit > 0) {