sysutils/cpupdate: - correct ONLY_FOR_ARCHS_REASON message;

• be more specific in the COMMENT and pkg-descr: mention x86-centricity;

• in addition to CPM option, add INTEL convinience option to ease

downloading and extraction of recently updated Intel's pack of microcode updates
https://downloadmirror.intel.com/27591/eng/:intel/microcode-20180312.tgz
allowed for end users but not for redistribution, so it does not get
installed nor added to pkg-plist.

Still no real changes of the package, so no PORTREVISION bump yet.