README: Refactor

* Use maximum 80 columns per line
* Add contents
* Add "Contribution" section

Signed-off-by: Andrei Gherzan <andrei@gherzan.ro>
linux: Add a Linux 3.6 kernel recipe

* Add a recipe to build the linux kernel from the rpi-3.6.y branch at
  https://github.com/raspberrypi/linux
* Since this has a higher version, it will be the preferred linux kernel
  by default.

Signed-off-by: Jan Schmidt <thaytan@noraisin.net>
bcm2835-bootfiles: Bump the firmware revision

* Pull in a newer firmware revision (0ac68c 7th Jan 2013) from
  https://github.com/raspberrypi/firmware for various fixes and
  improvements

Signed-off-by: Jan Schmidt <thaytan@noraisin.net>
sdcard-image: Use the size of the generated rootfs

When constructing the SD card image, the code was using
the inherited ROOTFS_SIZE, which is the size of the
rootfs contents. When building (for example) a compressed
rootfs, this allocates a partition much larger than necessary.

Instead, take the size of the generated rootfs file that is
about to be written into the generated image.

Signed-off-by: Jan Schmidt <thaytan@noraisin.net>