u-boot-script-boundary: Fix build for imx6qsabrelite

When MACHINE was set to imx6qsabrelite no bootscript.txt was found.
Fix the same for the 6x_upgrade script while at it. There is only
one upgrade script shared by all boards, so use it independent of
the MACHINE.

Signed-off-by: Christoph Settgast <christoph.settgast@methodpark.de>
Tested-By: Eric Nelson <eric.nelson@boundarydevices.com>
Acked-By: Eric Nelson <eric.nelson@boundarydevices.com>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
nitrogen6x: set KERNEL_IMAGETYPE to uImage

Signed-off-by: Eric Nelson <eric.nelson@boundarydevices.com>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
nitrogen6x-lite: set KERNEL_IMAGETYPE to uImage

Signed-off-by: Eric Nelson <eric.nelson@boundarydevices.com>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>