sdcard_image-rpi.bbclass: Allow optional compression

Image files will typically contain lots of null blocks and should compress well.
This will help with distribution of images over slow network links.

Change-Id: Icec7454ff61cd81a2872037037a93ce01191ece5
Signed-off-by: Paul Barker <paul@paulbarker.me.uk>
README: Fix IMAGE_FSTYPES example

The SD image is always created with the extension "rpi-sdimg" and this cannot be
changed by editing IMAGE_FSTYPES.

Change-Id: I7336e8c698f13fba14ef9cc9bf6c0b262bf261e5
Signed-off-by: Paul Barker <paul@paulbarker.me.uk>
Merge "sdcard_image-rpi.bbclass: Allow optional compression"
README: Add info

* Add some other minor formatting changes.
* Add info about Trello board.
* Add info about Gerrit server.
* Split source code info in a separate section.

Change-Id: Ib8fe5e11c1d95d14ff825a26d356b5ae432f2f93
Signed-off-by: Andrei Gherzan <andrei@gherzan.ro>
Merge "README: Add info"
Merge "README: Fix IMAGE_FSTYPES example"