bootutil: Fix AES and SHA-256 contexts not zeroized with mbedTLS

For some reason, the calls to mbedtls_aes_free, mbedtls_nist_kw_free and
mbedtls_sha256_free_drop were commented out which means the AES and
SHA-256 contexts were not properly de-initialized after usage when
mbedTLS is used. In the case of AES-KW it seems that might lead to a
memory leak depending on the mbedTLS configuration, but in any case and
independently of the mbedTLS configuration, this leads to the contexts
not be zeroized after usage.

Not zeroizing a context means it stays in RAM an undefined amount of
time, which might enable an attacker to access it and to dump the
sensitive data it contains.

Signed-off-by: Thomas Altenbach <thomas.altenbach@legrand.com>