staging: bcm2835_camera: Use a mapping table for context field of mmal_msg_header

staging: bcm2835_camera: Use a mapping table for context field of mmal_msg_header

The camera driver passes messages back and forth between the firmware with
requests and replies.  One of the fields of the message header called
context is a pointer so the size changes between 32 bit and 64 bit.

The context field is used to pair reply messages from the firmware with
request messages from the kernel.  The simple solution would be
to use the padding field for the upper 32 bits of pointers, but this
would rely on the firmware always copying the pad field.

So instead handles are generated that are 32 bit numbers and a mapping
stored in a btree as implemented by the btree library in the kernel lib
directory.  The mapping pairs the handle with the pointer to the actual
data. The btree library was chosen since it's very easy to use and
red black trees would be overkill.

The camera driver also now forces in the btree library if the camera is
included in the build.  The btree library is a hidden configuration
option.

Signed-off-by: Michael Zoran <mzoran@crowfest.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>