px4_mkfw.py: fix byte to string conversion

Tools/px_mkfw.py

@@ -101,11 +101,11 @@ if args.description != None:
 if args.git_identity != None:
 	cmd = "git --git-dir '{:}/.git' describe --exclude ext/* --always --tags".format(args.git_identity)
 	p = subprocess.Popen(cmd, shell=True, stdout=subprocess.PIPE).stdout
-	desc['git_identity']	= str(p.read().strip())
+	desc['git_identity']	= p.read().strip().decode('utf-8')
 	p.close()
 	cmd = "git --git-dir '{:}/.git' rev-parse --verify HEAD".format(args.git_identity)
 	p = subprocess.Popen(cmd, shell=True, stdout=subprocess.PIPE).stdout
-	desc['git_hash']	= str(p.read().strip())
+	desc['git_hash']	= p.read().strip().decode('utf-8')
 	p.close()
 if args.parameter_xml != None:
 	f = open(args.parameter_xml, "rb")