This commit fixes 'Impersonation in Passkey entry protocol'
(CVE-2020-26558) and suggests fixes for other vulnerabilites like
'Impersonation in the Pin Pairing Protocol' (CVE-2020-26555) and
'Authentication of the LE Legacy Pairing Protocol'
CVE-2020-26558 can be easily implemented if the peer device can
impersonate our public key. This commit adds a check by comparing our
and received public key and returns failed pairing if keys are same.
This commit also adds comments suggesting to use secure connection when
supported by all devices.
- Backport of IDF MR!6122.
- Modifies `smp_utils.c` to add check on encryption key size received from
peer.
- Modifies `esp_ble_gap_set_security_param` API to add minimum encryption key
size requirement.
