scripts: remove ZEPHYR_BASE as default root for board listing

Remove ZEPHYR_BASE as default root in list_boards.py.

This allows list_boards.py to be used to only print boards at given
root(s) without printing Zephyr default boards.

Secondly it remove the need in list_boards.py to have any knowledge of
ZEPHYR_BASE.

The `west boards` command already has ZEPHYR_BASE knowledge and can
easily add ZEPHYR_BASE to the list of roots it is already constructing,
thus removing the need for knowing that ZEPHYR_BASE is always added.

Update boards.cmake to pass ZEPHYR_BASE as an additional arch root, as
arch.cmake has not yet been processed,which means ZEPHYR_BASE is missing
in ARCH_ROOT list at this point in time.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>