toaster: toaster oe-selftest support

This patch adds toaster tests using the oe-selftest infrastructure.
You need to have builds done - the tests will verify data integrity
after the toaster collection phase.

Once you have your toaster builds done, to run the automated backend
tests via oe-selftest do the followings:

1. Update builddir/conf/bblayers.conf to contain the meta-selftest
layer
2. From the builddir run:

'oe-selftest toaster'

or if you just want to run a single test:

'oe-selftest toaster.Toaster_DB_Tests.testname'

This first part adds the meta/lib/oeqa toaster file.

Signed-off-by: Ionut Chisanovici <ionutx.chisanovici@intel.com>
Signed-off-by: Alexandru DAMIAN <alexandru.damian@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>