BuildBootImage: Add secondstage image support [DO NOT MERGE]

The AOSP bootimage format allows the use of a second stage image
however the BuildBootableImage function does not allows the "second"
optional argument. This patch adds the support of this argument.

Bug: 17035158

Change-Id: I8ed9d9e56449945c2d42fc908269921c394f68c0
Signed-off-by: Benoit Fradin <benoit.fradin@intel.com>
Signed-off-by: Jeremy Compostella <jeremy.compostella@intel.com>
Signed-off-by: Patrick Tjin <pattjin@google.com>

tools/releasetools/common.py

@@ -303,6 +303,11 @@ def BuildBootableImage(sourcedir, fs_config_file, info_dict=None):
 
   cmd = [mkbootimg, "--kernel", os.path.join(sourcedir, "kernel")]
 
+  fn = os.path.join(sourcedir, "second")
+  if os.access(fn, os.F_OK):
+    cmd.append("--second")
+    cmd.append(fn)
+
   fn = os.path.join(sourcedir, "cmdline")
   if os.access(fn, os.F_OK):
     cmd.append("--cmdline")