remove the need for pb2 subdir for pb2py generator

tools/build_pb.sh

@@ -1,5 +1,7 @@
 #!/bin/bash
 CURDIR=$(pwd)
+mkdir -p $CURDIR/pb2/
+echo > $CURDIR/pb2/__init__.py
 
 INDEX=../src/trezor/messages/wire_types.py
 rm -f $INDEX
@@ -8,8 +10,12 @@ for i in types messages storage ; do
     # Compile .proto files to python2 modules using google protobuf library
     cd $CURDIR/../../trezor-common/protob
     protoc --python_out=$CURDIR/pb2/ -I/usr/include -I. $i.proto
+done
 
+for i in types messages storage ; do
     # Convert google protobuf library to trezor's internal format
     cd $CURDIR
     ./pb2py $i ../src/trezor/messages/ $INDEX
 done
+
+rm -rf $CURDIR/pb2/

tools/pb2/.gitignore

@@ -1,2 +0,0 @@
-*_pb2.py
-*.pyc