oftree: add of_fix_tree() ...

Currently we have of_get_fixed_tree() which assumes that
there is a global variable holding a oftree. This seems
not very flexible, so for now introduce a of_fix_tree()
which takes an fdt_header as argument and walks the
list of registered of fixup functions.

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

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>