initial oftree command support ...

This adds basic device tree command support. So far we can
parse a flat device tree (-p), which also stores the tree
in memory, dump it (-d) and free (-f) the internally stored tree.

The chosen node can be updated with barebox bootargs, no other
device tree manipulation is implemented yet.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>