base.bbclass: warn when there is trailing slash in S or B variables

* to make sure we won't hit such corner cases in future add a warning
  to prevent trailing slashes and duplicated slashes even when they
  in most cases don't cause harm

* only a few cases were found in layers included in my world builds:
  oe-core: 1
  meta-oe: 7
  meta-python2: 1
  meta-qt5: 1
  meta-aws: 1

  will send patches for these once this warning is approved for oe-core

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

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