Makefile: put gofmt in diff mode for fmt target
Makefile: put gofmt in diff mode for fmt target

This also eliminates the need for two invocations of gofmt, and passes
src instead of the discrete packages as gofmt is recursive.

AlexNPavel/ignition,jonboulle/ignition,marineam/ignition,ajeddeloh/ignition,coreos/ignition,ajeddeloh/ignition,dgonyeo/ignition,dgonyeo/ignition,coreos/ignition,higebu/ignition,crawford/ignition,jonboulle/ignition,higebu/ignition,coreos/ignition,crawford/ignition,higebu/ignition,trnubo/ignition,mischief/ignition,jonboulle/ignition,ajeddeloh/ignition,mischief/ignition,AlexNPavel/ignition,dgonyeo/ignition,trnubo/ignition,trnubo/ignition,mischief/ignition,marineam/ignition,marineam/ignition,AlexNPavel/ignition