add LINUX_OSRELEASE to bsd.linux-rpm.mk

[John E Hein]

Boris Samorodov wrote at 21:34 +0400 on Apr  1, 2008:
 > > I've always disliked specifying the package file format in the linux
 > > .mk file name.
 > 
 > The thing is that linux-rpm.mk will stay and a new one should be
 > introduced for other purpose (i.e. to decide when to use different
 > linux infrastructure ports).

Understood.  I still don't like the name, but I can live with it ;)

I think there's too much stuff in there that has nothing to do with
rpm-ness.  And there's rpm-ness that is not necessarily specific to
linux-ness (although for all practical purpose I doubt there will ever
be a significant need to support rpm on a non-linux platform, so that's
a minor quibble).