Add image class

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Recipe for barebox

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
recipe for linux

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
recipe for imx-bootlets

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
mx28-bcb: add utility to generate Boot Control Block

From chapter 12.11.1 of the i.MX28 Applications Processor Reference
Manual, Rev. 1, 2010, a Boot Control Block is needed when booting from
an SD/MMC card. The mx28_bcb.py utility will generate and write the BCB
to a file, it will also write the provided bootstream.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>