conf/bitbake.conf: allow cache reuse between bitbake, bitbake-layers and hob

conf/bitbake.conf: allow cache reuse between bitbake, bitbake-layers and hob

BB_ENV_EXTRAWHITE is set by the bitbake wrapper script but not by
bitbake-layers, and thus it was affecting the data hash and thus
invalidating the cache. Any variables passed through from the
environment by being added to this variable should themselves
influence the hash already if they are not already excluded from,
the data hash, so BB_ENV_EXTRAWHITE can be excluded.

Similarly, DISABLE_SANITY_CHECKS is set by the hob script in order to
have sanity checks run later, but not set by the bitbake wrapper script.
It doesn't actually affect the build itself and so its value can also
be excluded from the data hash.

These changes prevent unnecessary reparsing between executions of
bitbake, bitbake-layers, and the initial pseudo build within the hob
script.

Fixes [YOCTO #2680].

(From OE-Core rev: 84bcacd5b4737520f0fc76333f0165ef5b87c207)

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