autogen.sh: make building from release tags work

This adapts Brent's commit
libressl/portable@c95574b
to rpki-client. OPENBSD_BRANCH can contain a branch, a tag, or a commit
hash from the rpki-client-openbsd repo, and git will do the right thing.

To make this actually work, we will need to create a release commit
where OPENBSD_BRANCH contains 'rpki-client-9.7'. This commit could
happen on master if we want to insist on a linear history or it could
live on a dedicated branch. I don't have a particular preference here
except that extra branches could be backported to earlier releases if
we care to do so.

Fixes rpki-client#120 (starting with release 9.7)

Author: botovq

OPENBSD_BRANCH

@@ -0,0 +1 @@
+master

update.sh

@@ -1,7 +1,7 @@
 #!/bin/sh
 set -e
 
-openbsd_branch=master
+openbsd_branch=$(cat OPENBSD_BRANCH)
 if [ -n "$1" ]; then
 	openbsd_branch="$1"
 fi
@@ -19,7 +19,8 @@ if [ -d openbsd/.git ]; then
 	(cd openbsd
 	 git fetch
 	 git checkout "${openbsd_branch}"
-	 git pull --rebase)
+	 # do not git pull on an rpki-client-X.Y release tag
+	 [ "${openbsd_branch%-*}" = rpki-client ] ||  git pull --rebase)
 fi
 
 # setup source paths