package_*.bbclass: Ensure OVERRIDES doesn't change sstate signature

Changes to OVERRIDES (e.g. from changing MACHINE) should not change
the sstate signatures of do_package_write_*. Exclude the variable
explicitly in the package classes to avoid this.

(From OE-Core rev: 5826a9260138c437f87ba1a9f84d5c08442b997d)

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