Fix the issue 'branch already exist' in rebuilds

[pavel-shirshov]

- What I did
Fixed Makefile of FRR. Before we had issues after #3589:

1. When you want to rebuild frr with new changes you get error "branch frr/7.1 is already exist".
2. When your patch list is empty stg undo gives an error

- How I did it

1. checkout to master branch after build.
2. Using temporary stg branch with timestamp
3. ignoring result code of stg undo

- How to verify it
make target/debs/stretch/frr_7.1-sonic-0_amd64.deb must work for you first
then
make target/debs/stretch/frr_7.1-sonic-0_amd64.deb-clean and
make target/debs/stretch/frr_7.1-sonic-0_amd64.deb must work for you again

- Description for the changelog

- A picture of a cute animal (not mandatory but encouraged)