Make "boot" a synonym for "imgexec", to match grub

src/hci/commands/image_cmd.c

 		.name = "imgexec",
 		.exec = imgexec_exec,
 	},
+	{
+		.name = "boot", /* synonym for "imgexec" */
+		.exec = imgexec_exec,
+	},
 	{
 		.name = "imgstat",
 		.exec = imgstat_exec,