In addition to the ${PROCESSOR} environment variable set by the [virtual]) attribute, there are environment variables you can set in the script portion of your buildfile.
You can use MKIFS_PATH to set the path where mkifs looks for the files it places in the image it builds. (See “Including files in the image” in this chapter.) Or, if you set the value of PATH in the boot file, procnto sets the _CS_PATH configuration string. Similarily, if you set LD_LIBRARY_PATH, procnto sets the _CS_LIBPATH configuration string. It doesn't pass these environment variables on to the script, but you can set environment variables in the script itself (see “Setting PATH and LD_LIBRARY_PATH” in the QNX Neutrino User's Guide).