ucslugc, nslu2: formalise different build configs, move to IAL 2.0 and nslu2-kernel

ucslugc, nslu2: formalise different build configs, move to IAL 2.0 and nslu2-kernel
  - ucslugc is substantially as before, however the configuration is now
  - expressed as a specific instance of nslu2-btu.conf (big-endian, thumb,
  - uclibc).  The other six buildable variants of ucslugc have been added
  - as separate configuration files, all with names of the form 'nslu2-???'
  - for big vs little, arm vs thumb and glibc vs uclibc.
  - IAL 2.0 is now in a testable state (reboot works) and so is the default
  - for all nslu2-??? configurations.
  - Overridable variables of the form UCSLUGC_* have been replaced with
  - variables named NSLU2_*