selftest: Add argument to keep build dir

The oe-selftest code already keeps the selftest build directory in place
if any tests failed. By default the build directory is deleted if all
tests pass but there may be cases where it's desirable to keep this
directory around, for example to compare intermediate files between
passing and failing test runs.

Signed-off-by: Paul Barker <pbarker@konsulko.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 67aa7069)
Signed-off-by: Steve Sakoman <steve@sakoman.com>