lib/oe/lsb: enable getting distro ID when lsb_release is not installed

lib/oe/lsb: enable getting distro ID when lsb_release is not installed

If lsb_release is not installed (as it may not be on headless/minimal
installations on distros whose LSB package has a long list of
dependencies) we need to gather the information directly from files in
/etc.

Fixes [YOCTO #4012].

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>