Revised U-boot altbootcmd

The hardcoded altbootcmd currently assumes that the first image that should be attempted is at offset 0x30000 in norflash and that the second is at 0x60000. It would be more robust if if first examined the bootfile variable to determine which offset contains the primary OBCSW image, and then attempted to boot the secondary first instead. This would prevent a boot loop where U-boot's primary boot command continues to boot 0x30000, there is something wrong with that image so the watchdog barks, and then the altbootcommand again tries to boot 0x30000 which fails instead of trying 0x60000.

If the altbootcmd is updated, not that it should also be updated in U-boot here! U-boot can then be uploaded to the norflash using the normal software update procedure.