sstate.bbclass: Add a string representing the lsb release to native/cross sstate

sstate.bbclass: Add a string representing the lsb release to native/cross sstate

This patch adds the lsb name and revision to the path used for sstate files.
This means that reuse of sstate files between different distributions is restricted
by default. The behaviour can be configured using mirror urls, for example:

SSTATE_MIRRORS = "file://Ubuntu-11.10/(.*) file://Ubuntu/\1 \n"

would map Ubuntu 11.10 to a more generic "Ubuntu" named sstate feed.

Usually, more modern distros have increased libc versions for example
so whilst more older native/cross sstate packages will usually work on newer
distros, the opposite is not true. This patch allows development of policy
to better handle this although no default policy is currently being used.

(From OE-Core rev: 9360fc318cec5c873b17f59b817dd6312c81ee27)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>