lib/oe/lsb.py: fall back to /etc/os-release for host distro ID

lib/oe/lsb.py: fall back to /etc/os-release for host distro ID

The new standard for host distribution identification [1] is
/etc/os-release, and a number of newer distributions provide this file,
so add support for this in order to pick up more distributions.
Additionally, handle "rolling release" style distributions that don't
report a version number, e.g. Arch Linux.

With this change we can identify the most common distributions, so this
should satisfy [YOCTO #4271]. Note that this doesn't imply support for
these distros as build hosts, just that we can identify them.

[1] http://www.freedesktop.org/software/systemd/man/os-release.html

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>