oeqa/sstatetests: Add NATIVELSB sstate signature equivalence test

oeqa/sstatetests: Add NATIVELSB sstate signature equivalence test

The sstate checksums should be independent of whichever NATIVELSBSTRING is
detected. Add an automated QA test which tests this using bitbake -S.

To make this possible, we need to be able to override the value of
NATIVELSBSTRING so make a small change to allow this.

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