sanity.bbclass: allow minimisation of impact of more invasive sanity checks

sanity.bbclass: allow minimisation of impact of more invasive sanity checks

Add a mechanism to avoid running more invasive checks every time bitbake is
invoked. Adds a file in conf/ (i.e. under user's build directory) that
tracks a new variable SANITY_VERSION (set in sanity.conf) as well as TMPDIR
and SSTATE_DIR, allowing checks to be run when these are changed.

Fixes [BUGID #562]

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