Config.in.legacy: adding renamed BR2 Options

Related to the changed directfb variables for
BR2_PACKAGE_DIRECTFB_DITHER_RGB16 and
BR2_PACKAGE_DIRECTFB_TESTS
adding a legacy check.

[Peter: fix DIRECTB_TESTS typo]
Signed-off-by: Carsten Schoenert <c.schoenert@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>