Fix several usability problems in the Makefile
Fix several usability problems in the Makefile

Allow "make run" and "make gdb" to be used on Linux, not just
non-Linux.

Allow the user to set the go binary from the commandline.

Pass GOROOT in to make different Go installs use the right
Go root.

Check that xorriso is installed, because if it is not, grub-mkrescue
will silently exit.

achilleasa/gopher-os,achilleasa/gopher-os