This commit adds a minimal test configuration `baremetal.h` to the `configs`
folder supporting ECDHE-ECDSA-AES-CCM-8 with Secp256R1 and SHA-256 only.
The configuration lacks some options which are currently needed to
successfully build and run the example applications `ssl_client2`
and `ssl_server2`, such as `MBEDTLS_NET_C`. To still allow testing
a configuration close to `baremetal.h`, the commit also adds
`baremetal_test.h`, containing minimal amendments to `baremetal.h`
that allow building and running `ssl_client2` and `ssl_server2`.
