Revert "core: enable 'NDEBUG' unless BR2_ENABLE_RUNTIME_DEBUG is set"

Revert "core: enable 'NDEBUG' unless BR2_ENABLE_RUNTIME_DEBUG is set"

Enabling -DNEBUG, although correct on the paper, causes a lot of
packages to fail to build because they explicitly require not building
with NDEBUG; they use assert() to check actual runtime errors and expect
it to not be elidded away (sometimes with side effects in the arguments
passed to assert().

This reverts commit 5a8c50fe05afacc3cbe8e7347e238da9f242fab0, as
discussed on the list:
    http://lists.busybox.net/pipermail/buildroot/2021-July/313646.html

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>