utils/prepare-mode.sh: API change and clarification.

Variables controlling export and arch changed to:

* BUILD_CPU - CPU architecture to use, in format of '-march=$BUILD_CPU'.
It is orthogonal to selecting build mode, and is useless without build
mode selection.

* BUILD_MODE - gets "export" and "import" values which make use of
BUILD_CPU setting. "export" activates NO_INSTALL, sets PKGDEST and build
arch, while "import" only sets PKGDEST.