base platforms variable removal

[Joshua Root]

On 2017-10-8 08:56 , Kurt Hindenburg wrote:
> Hello,  from the guide and the source code, it appears that the Portfile
> platforms variable is not used at all.  As such, I’m unclear why lint
> says it is an error to not have it.  Regardless, if there’s no use for
> it now or in the future, I could put together a PR to remove it from
> base.  Then at a later time, it could be removed from Portfiles.

It is informational like several other variables such as categories. It 
has been proposed to extend platforms to indicate which OS version a 
port does and does not work on, and use that to skip builds on the 
buildbot as well as replace all the manual 'return -code error "This 
port doesn't work on Tiger"' sort of stuff.

- Josh