cmd/fyne: Improve consistency in cli parameters

[Jacalz]

Discussed on the call today. We need to make sure that parameters to different commands are both consistent and make sense. One potentially strange example is how target and os are the same thing. Which one is the most like what users would expect? We are doing are first release so removing or changing parameters is fine.

tools/cmd/fyne/internal/commands/build.go

Lines 51 to 58 in 9dc8796

	Name: "target",
	Aliases: []string{"os"},
	Usage: "The mobile platform to target (android, android/arm, android/arm64, android/amd64, android/386, ios, iossimulator).",
	Destination: &b.os,
	},
	&cli.StringFlag{
	Name: "sourceDir",
	Aliases: []string{"src"},

cc @sdassow