cmake: improve arch error message for invalid arch selection

Legacy hw model picked the arch based on folder names.
The new hw model allows for greater flexibility and instead uses the
architecture defined by `CONFIG_ARCH` in Kconfig.

Therefore check that ARCH is defined and fail with a better error
message if ARCH is not defined. The error message includes the board and
SoC selected to make it easier to trace such errors.

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