Linux icons are now only copied for build process, not for install
process. They should be ignored for all platforms except RPM package
build on Linux.
This is so that the atom.desktop file will be able to find the
"atom" icon when requested. This adds the dependency of ImageMagick
to convert atom.png to varying resolutions, although, only during
the rpm build process (not during actual install). So the result
is ultimatly no different for the end user.
Also, fixed an absolute path from the rpm build process and made
it relative. It was hardcoded in the spec file to
"/tmp/atom-build/Atom/*", so builds that were made elsewhere would
have broken when attempting to package into an rpm. Now rpm packaging
should work from a build made anywhere. Also needed to modify
script/mkrpm so that it copies the build files in such a way that
they can more easily be dealt with in the spec file in a relative
way.
