gpio: fix a bug which prevented digital pins reads from working correctly

when reading from the value file, it is important to seek back to the front
so that we get the fresh values from the kernel driver for every new read
gpio: remove edge file handling from digital pin

to be handled in future
samples: enable flag parsing

this will allow people to use glog options (like -v=3) to increase
verbosity of the log output
bbb: initialise at the start of SetMicroseconds

this will setup p.period to the default if not already done
us020: make the state comparisons clearer