linux 2.6.28: fix nokia900 build

linux 2.6.28: fix nokia900 build

- defconfig is based on Maemo5 one (present in sources as rx51_defconfig)
- MACHINE_KERNEL_PR is set in nokia900.conf as we do not use linux-omap
  recipes for that device. It was discussed with OMAP3 maintainter (Koen
  Kooi) on IRC:

11:02 < hrw> koen: nokia900 uses omap3.inc and I need to bump kernel - are you ok with bumping M_K_PR for all omap3?
11:02 < hrw> koen: I can also move M_K_PR to nokia900.conf to separate it
11:02 < hrw> this device do not use linux-omap recipes
11:02 < hrw> at least not yet
11:06 < _koen_> hrw: either solution is fine by me, but if it isn't using linux-omap I think I prefer moving MKPR to nokia900.conf