cmake: app: set zephyr_extra_modules from environment

This PR adds the possibility to specify ZEPHYR_EXTRA_MODULES from an
environmental variable. To add a custom module mymodule in
path/mymodule the variable ZEPHYR_EXTRA_MODULES can be set in
`.zephyrrc` as `export ZEPHYR_EXTRA_MODULES=path/mymodule`.

Signed-off-by: Laczen JMS <laczenjms@gmail.com>