Cosmetic fix, no functional change. A little less duplication and
hardcoding.

listdir_at() had an optional "encoding" parameter but it was never used
before now, always defaulting to 'utf-8'.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>

No functional change: just a lot less duplication & hardcoding.

This can help testing and supporting other encodings. It does not hurt
in any case.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>

Fixes issue reported in PR zephyrproject-rtos#710 where most places are hardcoded to
'utf-8' while this one is (Windows) locale-dependent. In the future, we
may want to make this more flexible but the most urgent fix is
consistency: with this commit, manifest decoding should be hardcoded to
'utf-8' everywhere.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>