Linux specifies the linux,stdout-path property in the /chosen node
in the devicetree. Unfortunately this is ignored in most cases.
For cases in which barebox uses this property for its own use we
translate this into a Linux boot arg with:

- the console name provided by the serial driver
- the the instance from the 'serial' alias
- the baudrate from the actual baudrate.

So with this it's for devicetee enabled boards no longer necessary
to manually assign a console= parameter. Should a user not want
to use the automatically assigned parameter it should do:

global.linux.bootargs.console=

in the environment.

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