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-21.02" branch,
version computation is wrong, because the number of local commits is
computed against master.  As a result, it wrongly counts *all* commits
since the beginning of the openwrt-21.02 branch as local commits.

The fix is to compare to the openwrt-21.02 branch instead, which gives the
expected result.

A similar change had been applied to the openwrt-19.07 branch:
891022918d55 ("scripts: getver.sh: fix version based on stable branch")

Signed-off-by: Baptiste Jonglez <git@bitsofnetworks.org>