[MACHINEOVERRIDES][FIX] Adopted usage of more general seco-arm
machineoverride
Where is present a conditional metadata operation (selecting a variable) that involves all ARM architecture, the seco-arm label is used, taking the place of specific ones (e.g. seco-im6, seco-imx8, rockchip).
This fixs the error in using a list of machine overrides in as conditional metadata operation.