[CMake] CPack RPM generator not using CPACK_RPM_<component>_XXX variables

[Domen Vrankar]

> Can this use of mixed case-sensitive/all-upper-case can be clarified in the
> documentation for CPACK_ and CPAC_RPM_ component variables?

I was certain that this was already documented... I'll document it and
also change CPackRPM so that it will support both original version of
component name and upper cased version so there will be less
confusion.

Thanks,
Domen