[buildbot] phase1: run maketag.sh under exclusive lock

[LEDE Commits]

jow pushed a commit to buildbot.git, branch master:
https://git.lede-project.org/d2423387dc7babed61d1bc152784d300ce3a6d0e

commit d2423387dc7babed61d1bc152784d300ce3a6d0e
Author: Jo-Philipp Wich <jo at mein.io>
AuthorDate: Wed Dec 14 01:26:08 2016 +0100

    phase1: run maketag.sh under exclusive lock
    
    Signed-off-by: Jo-Philipp Wich <jo at mein.io>
---
 phase1/master.cfg | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/phase1/master.cfg b/phase1/master.cfg
index e73f31d..dc1e66d 100644
--- a/phase1/master.cfg
+++ b/phase1/master.cfg
@@ -206,6 +206,7 @@ def GetVersionPrefix(props):
 c['builders'] = []
 
 dlLock = locks.SlaveLock("slave_dl")
+tagLock = locks.MasterLock("make_tag")
 
 checkBuiltin = re.sub('[\t\n ]+', ' ', """
 	checkBuiltin() {
@@ -309,7 +310,8 @@ for target in targets:
 		path = home_dir+'/source.git',
 		env = {'GNUPGHOME': gpg_home},
 		haltOnFailure = True,
-		doStepIf = IsTaggingRequested
+		doStepIf = IsTaggingRequested,
+		locks = [tagLock.access('exclusive')]
 	))
 
 	# switch to branch