copy_buildsystem: allow more layer paths

Layers could be located anywhere. The eSDK should work with them even if
they are not located in TOPDIR or in the same parent directory as
COREBASE.

For layers located in the same parent directory as COREBASE this preserves
the intent from the previous
  copy_buildsystem: include layer tree during build structure creation
commit.

Related OE-Core rev: 5a59a699

Signed-off-by: Daniel Wagenknecht <dwagenknecht@emlix.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>