Makefile: clean MatrixVision builds

"make clean" after builds of MatrixVision boards would leave stale
files around:

	board/matrix_vision/mvblm7/bootscript.img
	board/matrix_vision/mvsmr/bootscript.img

Fix this.
Signed-off-by: NWolfgang Denk <wd@denx.de>

Makefile

@@ -2458,6 +2458,7 @@ clean:
 	       $(obj)tools/mkimage	   $(obj)tools/mpc86x_clk	  \
 	       $(obj)tools/ncb		   $(obj)tools/ubsha1
 	@rm -f $(obj)board/cray/L1/{bootscript.c,bootscript.image}	  \
+	       $(obj)board/matrix_vision/*/bootscript.img		  \
 	       $(obj)board/netstar/{eeprom,crcek,crcit,*.srec,*.bin}	  \
 	       $(obj)board/trab/trab_fkt   $(obj)board/voiceblue/eeprom   \
 	       $(obj)board/armltd/{integratorap,integratorcp}/u-boot.lds  \