set ( $ENV{ZEPHYR_EXTRA_MODULES} pbs_path/module1)
Looking at this quickly, ZEPHYR_EXTRA_MODULES needs to be set before including the boilerplate file, not before. You can also set it as an ordinary CMake variable; you don't need to set it in the environment.
Unfortunately, with this set up module1 doesn't get compiled and I get a linker error when I try to use functions from that module. I assume that something goes wrong with ZEPHYR_EXTRA_MODULES but I don't know what.