'make flash' is failing for altera_max10 board due to the
missing NIOS2_CPU_SOF environment variable. Though it is set
in arch/nios2/soc/nios2f-zephyr/CMakeLists.txt but it is not
taking effect when flashing script is run. The reason could be
following which is mentioned in https://itk.org/Wiki/CMake_FAQ
"environment variables SET in the CMakeLists.txt only
take effect for cmake itself (configure-time), so you cannot use
this method to set an environment variable that a custom command
might need (build-time)."
Now, NIOS2_CPU_SOF is set from boards/nios2/altera_max10/board.cmake
file which is more logical because all the FLASH related environment
variables are being set from board.cmake
Signed-off-by: Ramakrishna Pallala <ramakrishna.pallala@intel.com>