boston: Enable CONFIG_DISTRO_DEFAULTS in defconfigs

CONFIG_DISTRO_DEFAULTS selects a number of things we want for Boston
defconfigs & generally describes what we want - to be able to boot an
arbitrary Linux distribution. Enable it in order to shorten the
defconfigs & to automatically keep up with any changes in the choice of
Kconfig symbols selected.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>