bitbake.conf: Add BB_HASH_CODEPARSER_VALS

Add an initial value for the new bitbake BB_HASH_CODEPARSER_VALS support,
which optimises the codeparser cache's size and stops it growing at every
parse.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>