cmake: create zephyr_build_string

This commit creates a new `zephyr_build_string()` function.

`zephyr_build_string()` provides a common way of constructing a build
string based on board name, board revision, and build type.

Having this as a function facilitate creation of names in a common way
and thus helps to ensure naming consistency, especially if newer info
is later embedded into the build string.

Initially the build string will be used to lookup configuration
fragments and devicetree overlay files.

This function is used by `zephyr_file(CONF_FILES ...)`.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>