Kconfig: Rename $ENV_VAR_{ARCH,BOARD_DIR} to $ARCH/$BOARD_DIR

The prefixes might be a leftover from the old 'option env="..."' symbols
(which are no longer needed). Since environment variables can be
referenced directly now, there's no point in having a prefix.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>