booting: more flexible Linux bootargs generation ...

We currently use the environment variable 'bootargs' to get the
Linux bootargs. This patch allows for a more flexible bootargs
generation using global variables. With it the Linux bootargs
are concatenated from multiple variables. This allows to replace
parts of the bootargs string without having to reconstruct it
completely.
With this bootargs can be constructed like:

global linux.bootargs.base="console=ttyS0,115200"
global linux.bootargs.ip="ip=dhcp"
global linux.mtdparts="physmap-flash.0:512K(nor0.barebox),-(root)"

This will then automatically be combined into a kernel bootargs
string during boot.

If the 'linux.bootargs.' variables are all empty the old standard
'bootargs' way will be used.

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

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>