doc: kconfig: Get rid of some duplicate path separators in output

Paths like soc//arm/... showed up in the output due to the extra '/' at
the end of the SOC_DIR value. Remove the redundant '/' in SOC_DIR and
other environment variables that get referenced in the Kconfig files.

Kconfiglib never normalizes paths, because it avoids some gotchas, e.g.
with symlinks.

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