board: em_starterkit doesn't have FLASH_SCRIPT yet

I see in the TOP makefile that if FLASH_SCRIPT is undefined,
it outputs the message "Flashing not supported with this board".
This behavior is wanted currently as this board does not
have a memory mapped FLASH device. It has a SPI-FLASH and
there are different instructions for putting an image on to the board.

Change-Id: I07dcdd9a7fd8346b846ee0fe1312d22f9ffaa13e
Signed-off-by: Chuck Jordan <cjordan@synopsys.com>