imx-base.inc: Allow PREFERRED_VERSION of xserver-xorg to be customized

For development and test it is useful to be able to override the Xorg
preferred version so we make it overridable.

Change-Id: I6bba649187d2d35ae1c92ce959d0f61802397423
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
gpu-viv-bin-mx6q.inc: Fix libopencl-mx6 runtime dependency

The libopencl-mx6 requires libclc-mx6 to proper run so we move it from
libopencl-mx6-dev package to libopencl-mx6.

Change-Id: I125a01022cf04f528a954cc4c1cbc826af79608e
Reported-by: Sébastien Taylor <me@staylor.ca>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
elftosb: 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 BCB_struct.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>