libweave: Rename buffet namespace into weave

Namespace renamed only in libweave dirrectory. Namespace of buffet/
stays same.

BUG=brillo:1235
TEST=FEATURES=test emerge-gizmo buffet

Change-Id: If45c63f443fab792b661ea85f57ddacc380d0a13
Reviewed-on: https://chromium-review.googlesource.com/284578
Reviewed-by: Vitaly Buka <vitalybuka@chromium.org>
Commit-Queue: Vitaly Buka <vitalybuka@chromium.org>
Tested-by: Vitaly Buka <vitalybuka@chromium.org>