[frr]: Fix the issue 'branch already exist' in frr package rebuilds (#3692)

Fixed Makefile of FRR. Before we had issues after #3589:

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

src/sonic-frr/Makefile

@@ -4,8 +4,9 @@ SHELL = /bin/bash
 
 MAIN_TARGET = $(FRR)
 DERIVED_TARGET = $(FRR_PYTHONTOOLS) $(FRR_DBG) $(FRR_SNMP) $(FRR_SNMP_DBG)
+SUFFIX = $(shell date +%Y%m%d\.%H%M%S)
 FRR_BRANCH = frr/$(FRR_VERSION)
-STG_BRANCH = stg_temp
+STG_BRANCH = stg_temp.$(SUFFIX)
 
 $(addprefix $(DEST)/, $(MAIN_TARGET)): $(DEST)/% :
 	# Build the package
@@ -15,11 +16,12 @@ $(addprefix $(DEST)/, $(MAIN_TARGET)): $(DEST)/% :
 	stg import -s ../patch/series
 	tools/tarsource.sh -V -e '-sonic'
 	dpkg-buildpackage -rfakeroot -b -us -uc -Ppkg.frr.nortrlib -j$(SONIC_CONFIG_MAKE_JOBS)
-	stg undo
+	stg undo || true
 	git clean -xfdf
 	git checkout $(FRR_BRANCH)
 	stg branch --delete $(STG_BRANCH)
 	git rev-parse --short HEAD | xargs git checkout
+	git checkout master
 	git branch -D $(FRR_BRANCH)
 	popd
 	mv $(DERIVED_TARGET) $* $(DEST)/