samples: bt: some RAM footprint reductions

As part of the work of deprecating TinyCrypt in favor of
PSA crypto API, this commit fixes some kconfigs in order to
reduce the RAM footprint. In all involved samples RAM usage
was extremely high even with TinyCrypt (usually > 98%) and
of course it got overflowed once TinyCrypt got replaced by
Mbed TLS's PSA Crypto API.

Signed-off-by: Valerio Setti <vsetti@baylibre.com>