selftest reproducible.py: support different build targets

Allow users to set different build reproducibility targets than
the defaults using OEQA_REPRODUCIBLE_TEST_TARGET and
OEQA_REPRODUCIBLE_TEST_SSTATE_TARGETS variables in local.conf.

Fixing all issues from "world" builds is not possible in some
complex build environments with lots of layers. Limiting the focus to
a smaller subset allows using this test to detect and fix build
reproduction issues incrementally.

Signed-off-by: Mikko Rapeli <mikko.rapeli@linaro.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>