[oe] base.bbclass: Enable overwrite flag for unzip command

[Rod Whitby]

Leon Woestenberg wrote:
> During rebuilds I noticed unpacking a zip a second time fails, whereas tar
> silently succeeds, overwriting files.
...
> I will shortly commit the following change (after a test), which enables
> overwrite behaviour for unzip:
> 
> -base.bbclass:574:               cmd = 'unzip -q'
> +base.bbclass:574:               cmd = 'unzip -q -o'

+1 - I had the same problem arise today with ixp4xx-npe-native.

-- Rod