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

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 <git@bitsofnetworks.org>
[shorten commit title]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>