cmake: calculate the md5 sum using the built-in string command

Assuming that all supported releases require at least CMake
version 3.0, we should be able to calculate the md5sum in a
much simpler way using string().
https://cmake.org/cmake/help/v3.0/command/string.html



This avoids writing a temporary file into the source directory
and therefore mitigates the possibility of the following
warning.

share/zephyr-package/cmake/zephyr_export.cmake:19 (string):
  string sub-command SUBSTRING requires four arguments.

Signed-off-by: Christopher Friedt <cfriedt@meta.com>