releasetools: Prefer the avbtool specified in target_files.

This allows a consistent logic in using the avbtool which could be
board-specific.

Test: `atest releasetools_test`
Test: Run sign_target_files_apks.py on a target_files.zip.
Change-Id: I8cd93b8e71146985734f85c31f4662f5e2e9534c

tools/releasetools/sign_apex.py

@@ -19,6 +19,9 @@ Signs a standalone APEX file.
 
 Usage:  sign_apex [flags] input_apex_file output_apex_file
 
+  --avbtool <avbtool>
+      Optional flag that specifies the AVB tool to use. Defaults to `avbtool`.
+
   --container_key <key>
       Mandatory flag that specifies the container signing key.
 
@@ -40,12 +43,14 @@ import common
 logger = logging.getLogger(__name__)
 
 
-def SignApexFile(apex_file, payload_key, container_key, signing_args=None):
+def SignApexFile(avbtool, apex_file, payload_key, container_key,
+                 signing_args=None):
   """Signs the given apex file."""
   with open(apex_file, 'rb') as input_fp:
     apex_data = input_fp.read()
 
   return apex_utils.SignApex(
+      avbtool,
       apex_data,
       payload_key=payload_key,
       container_key=container_key,
@@ -59,7 +64,9 @@ def main(argv):
   options = {}
 
   def option_handler(o, a):
-    if o == '--container_key':
+    if o == '--avbtool':
+      options['avbtool'] = a
+    elif o == '--container_key':
       # Strip the suffix if any, as common.SignFile expects no suffix.
       DEFAULT_CONTAINER_KEY_SUFFIX = '.x509.pem'
       if a.endswith(DEFAULT_CONTAINER_KEY_SUFFIX):
@@ -77,6 +84,7 @@ def main(argv):
       argv, __doc__,
       extra_opts='',
       extra_long_opts=[
+          'avbtool=',
           'container_key=',
           'payload_extra_args=',
           'payload_key=',
@@ -91,6 +99,7 @@ def main(argv):
   common.InitLogging()
 
   signed_apex = SignApexFile(
+      options.get('avbtool', 'avbtool'),
       args[0],
       options['payload_key'],
       options['container_key'],