lib/oe/lsb: better handle missing fields

lib/oe/lsb: better handle missing fields

Some rolling release distros, such as Arch Linux, don't include a
VERSION_ID field in their os-release file.

Change release_dict_osr() to better handle this optional field
being absent.

Further improve the resilience of the release_dict_*() methods by
always returning a dict and using dict.get() in distro_identifier()
to supply a default, empty string, value when then key is missing.

(From OE-Core rev: e36066dcc3b56cac1c695370ea178b566c0ebfd6)

Signed-off-by: Joshua Lock <joshua.g.lock@intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>