Sorted a variable initialisation issue that was giving incorrect results

Sorted a variable initialisation issue that was giving incorrect results
with wiringPiISR.
Added the ability to readall/reset from extension modules.
Changed the build script to use /bin/sh rather than /bin/bash as it seems
some installations don't have /bin/bash (or if they do, then the people who
are telling me that ./build gives them command not found are wrong)