board/BuR/common: refactor ft_board_setup(...)

On other OS, not one provided by B&R, it is not guaranteed that there
are factory-settings within a devicetree. So we must not treat the
absence of them as error.
Further we've the fact that on different version of the device-tree
files there are different namings of the factory-settings, we consider
this with searching for an alternative name.

changing things as following:

- don't treat as error if the bootloader version cannot written into
devicetree.

- since the naming of the factory-settings are different in different
versions of the provided device-tree we search for the alternate name
"/fset"

Signed-off-by: Hannes Schmelzer <oe5hpm@oevsv.at>