build: make board directory configurable

[MarcFinetRtone]

Since wm-sdk could be used as an external repository where code and
board configuration could be defined, and $(BOARDS_PATH) already
exists (as IMAGE_PATH or GLOBAL_BUILD), use it rather than using
the hardcoded 'board/'.

[GwendalRaoul]

Hello,

Thanks for the patch.
We also spotted the same issue (for internal usage needs) and similar modification is already in our internal code base.
So it will be seen on GitHub in our next release.
And there is already a preview in branch v1.2.3_beta1 : https://github.com/wirepas/wm-sdk/tree/v1.2.3_beta1

Best regards,
Gwendal