bitbake.conf: Exclude extend_recipe_sysroot function from sstate checksums

bitbake.conf: Exclude extend_recipe_sysroot function from sstate checksums

The extend_recipe_sysroot itself is excluded from the task hashes. This is
because it only ever acts upon the contents of the task dependencies and hence
those checksums accurately relfect what its doing. It does mean sysroots don't
repopulate if this function changes but there are other easy ways to achieve
that if needed.

(From OE-Core rev: cec305150801d43d58e3758e020e714d2e90e10b)

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