[gentoo-dev] [PATCH v2] Refactor virtual/opencl to provide the API, not an implementation

[Marek Szuba]

Changes since v1:
 * bump EAPI to 7
 * use readme.gentoo for the postinst message
 * do not depend on app-eselect/eselect-opencl
 * make note in the comments about dev-libs/opencl-icd-loader to explain
   why we need a virtual in the first place