scripts: getver.sh: fix version based on stable branch

When building from a local branch based off the "openwrt-18.06" branch,
version computation is wrong, for instance:

    r6907+1154-7e15e21766

The number of local commits (1154 in this case) is wrong because it is
computed against master.  As a result, it wrongly counts *all* commits
since the beginning of the openwrt-18.06 branch as local commits.

The fix is to compare to the openwrt-18.06 branch instead, which gives the
expected result such as:

    r8060+1-1238a22316

Signed-off-by: Baptiste Jonglez <[email protected]>
[shorten commit title]
Signed-off-by: Adrian Schmutzler <[email protected]>

scripts/getver.sh

@@ -26,7 +26,7 @@ try_git() {
 	*)
 		BRANCH="$(git rev-parse --abbrev-ref HEAD)"
 		ORIGIN="$(git rev-parse --verify --symbolic-full-name ${BRANCH}@{u} 2>/dev/null)"
-		[ -n "$ORIGIN" ] || ORIGIN="$(git rev-parse --verify --symbolic-full-name master@{u} 2>/dev/null)"
+		[ -n "$ORIGIN" ] || ORIGIN="$(git rev-parse --verify --symbolic-full-name openwrt-18.06@{u} 2>/dev/null)"
 		REV="$(git rev-list ${REBOOT}..$GET_REV | wc -l | awk '{print $1}')"
 
 		if [ -n "$ORIGIN" ]; then