u-boot-script-boundary: handle the case where MACHINE=imx6sabrelite

else the build fails with :
uboot-mkimage: Can't open board/boundary/imx6qsabrelite/6x_bootscript: No such file or directory

Signed-off-by: Eric Bénard <eric@eukrea.com>
Cc: Eric Nelson <eric.nelson@boundarydevices.com>