fastboot: Avoid extra std::string copies

The function do_for_partitions() takes a const std::string
reference, so it's inefficient to pass a std::string::c_str().

Test: TreeHugger
Change-Id: Ia84ed9ec691ee2f524c61dd25a81b2995bb0bb33
diff --git a/fastboot/fastboot.cpp b/fastboot/fastboot.cpp
index 4cdd8bc..827db96 100644
--- a/fastboot/fastboot.cpp
+++ b/fastboot/fastboot.cpp
@@ -1805,7 +1805,7 @@
             auto format = [&](const std::string& partition) {
                 fb_perform_format(partition, 0, type_override, size_override, "");
             };
-            do_for_partitions(partition.c_str(), slot_override, format, true);
+            do_for_partitions(partition, slot_override, format, true);
         } else if (command == "signature") {
             std::string filename = next_arg(&args);
             std::vector<char> data;
@@ -1876,7 +1876,7 @@
                 }
                 do_flash(partition.c_str(), fname.c_str());
             };
-            do_for_partitions(pname.c_str(), slot_override, flash, true);
+            do_for_partitions(pname, slot_override, flash, true);
         } else if (command == "flash:raw") {
             std::string partition = next_arg(&args);
             std::string kernel = next_arg(&args);