When supervisor cards are shipped out from the factory, the config-register is set to 0x2101.   The value of 0x2101 means "ignore the boot variable string and boot the first valid IOS image from flash".  

The correct config register value should be 0x2102 but this means you need to have the correct boot variable string or else the switch will either boot the wrong IOS file or the switch will go into ROMmon.