ar71xx: Move OpenMesh image target validation into subfunction

ar71xx: Move OpenMesh image target validation into subfunction

The platform_check_image_openmesh function used break statements to signal
that the board name matched the image target. This worked because the
sysupgrade binary checked the image inside a loop. The break statement
stopped the loop and skipped any additional check.

Instead the check should be done without such sideeffects by simply
combining the board names and image targets. Only a mismatch should cause a
negative result for the caller and skipping of the additional checks.

Signed-off-by: Sven Eckelmann <sven.eckelmann@open-mesh.com>