tcl/target: Abort on invalid SoC selection on R-Car Gen3

Instead of printing error message and continue, abort on invalid SoC
selection right away.

Change-Id: I9c7a7111b590c6c49a0826562380b881a162a8dc
Signed-off-by: Marek Vasut <marek.vasut@gmail.com>
Reviewed-on: http://openocd.zylin.com/5439
Tested-by: jenkins
Reviewed-by: Oleksij Rempel <linux@rempel-privat.de>

tcl/target/renesas_rcar_gen3.cfg

@@ -76,7 +76,7 @@ switch $_soc {
 		set _boot_core CA53
 	}
 	default {
-		echo "'$_soc' is invalid!"
+		error "'$_soc' is invalid!"
 	}
 }