phase1: do not leak targets between branches

phase1: do not leak targets between branches

Robert noticed, that after rename of `ipq807x` target in main branch to
`qualcommax/ipq807x` subtarget, that buildbot is trying to build this
new `qualcommax/ipq807x` subtarget on `openwrt-23.05` branch as well.

Thibaut later explained, that this is by design, his initial idea was to
find exhaustive list of all targets and let the `checkarch` step do the
final triaging.

I find this approach confusing, because if the subtarget is not present
in that branch, we shouldn't have builder for it configured as well.
Furthermore wasting roughly 5 minutes of precious buildworker time to
checkout all feeds and then just find out, that we're not going to use
those seems suboptimal as well.

So lets fix it by using builders for targets as present in respective
branches.

Fixes: #14
Reported-by: Robert Marko <robimarko@gmail.com>
Signed-off-by: Petr Štetiar <ynezz@true.cz>