bluetooth: host: smp: fix deadlock when public key generation fails

When `bt_le_oob_get_local` or `bt_le_ext_adv_oob_get_local` is called
and SMP is enabled, `bt_smp_le_oob_generate_sc_data` is called to
generate a Random Number and a Confirmation Value needed for OOB data.
These values are based on the device's public key.

The public key is generated only once when `bt_smp_init` is called.
If public key generation fails, the callback passed to `bt_pub_key_get`
is called with `pkey` set to NULL. The `bt_smp_pkey_ready` callback
gets called, but it doesn't release the `sc_local_pkey_ready` semaphore
thus leaving `bt_smp_le_oob_generate_sc_data` wait for semaphore with
`K_FOREVER`.

This commit replaces the semaphore with a conditional variable and
requests a public key again if the key is NULL thus solving 2 issues:
- handling the case where the callback was triggered notifying about the
  completion of the public key request, but the key was not generated,
- handling the case where multiple threads trying to acquire the same
  sempahore.

The timeout is used instead of K_FOREVER to avoid cases when callback
has never been triggered.

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>