main: change -json flag to match upstream Go

This changes the -json flag for build/run/flash/gdb etc commands to not
print `compileopts.Config` but instead match upstream Go and print build
errors in a structured way.

For more details, see the proposal:
https://github.com/golang/go/issues/62067

compileopts/options.go

@@ -53,7 +53,6 @@ type Options struct {
 	Programmer      string
 	OpenOCDCommands []string
 	LLVMFeatures    string
-	PrintJSON       bool
 	Monitor         bool
 	BaudRate        int
 	Timeout         time.Duration