base.bbclass: Add base_path_relative.

base.bbclass: Add base_path_relative.

base_path_relative returns a relative path from src to dest.

Examples:

>>> base_path_relative("/usr/bin", "/tmp/foo/bar")
../../tmp/foo/bar

>>> base_path_relative("/usr/bin", "/usr/lib")
../lib

>>> base_path_relative("/tmp", "/tmp/foo/bar")
foo/bar

Signed-off-by: Chris Larson <clarson@mvista.com>