Improve fastboot verbose output.

Move some verbose messages so they're only output with -v.

Remove some misleading verbose messages altogether (such as "wiping
userdata...").

Properly log all the commands sent and responses received. Example:

  $ fastboot -v flashall
  ...
  Sending sparse 'system' 2/2 (443712 KB)
  fastboot: verbose: sending command "download:1b1500d0"
  fastboot: verbose: received DATA 1b1500d0
  fastboot: verbose: sending data (1024 bytes)
  fastboot: verbose: sending data (165596160 bytes)
  fastboot: verbose: sending data (1024 bytes)
  fastboot: verbose: sending data (267762688 bytes)
  fastboot: verbose: sending data (1024 bytes)
  fastboot: verbose: sending data (20978688 bytes)
  fastboot: verbose: sending data (1024 bytes)
  fastboot: verbose: sending data (7168 bytes)
  fastboot: verbose: sending data (1024 bytes)
  fastboot: verbose: sending data (3072 bytes)
  fastboot: verbose: sending data (1024 bytes)
  fastboot: verbose: sending data (3072 bytes)
  fastboot: verbose: sending data (1024 bytes)
  fastboot: verbose: sending data (3072 bytes)
  fastboot: verbose: sending data (208 bytes)
  fastboot: verbose: received OKAY
  OKAY [ 13.871s]
  Writing sparse 'system' 2/2
  fastboot: verbose: sending command "flash:system"
  fastboot: verbose: received OKAY
  OKAY [  3.228s]
  Rebooting
  fastboot: verbose: sending command "reboot"
  fastboot: verbose: received OKAY

  Finished. Total time: 36.939s

Bug: http://b/30953083
Bug: http://b/74444116
Test: `fastboot -v flashall`
Change-Id: I2cca198daa062fd1fb732218343263803b111e38
diff --git a/fastboot/engine.cpp b/fastboot/engine.cpp
index 875e7b9..551ddba 100644
--- a/fastboot/engine.cpp
+++ b/fastboot/engine.cpp
@@ -313,6 +313,7 @@
         a->start = now();
         if (!a->msg.empty()) {
             fprintf(stderr, "%-50s ", a->msg.c_str());
+            verbose("\n");
         }
         if (a->op == OP_DOWNLOAD) {
             status = fb_download_data(transport, a->data, a->size);