boot: Move encryption context invalidation to boot_enc_drop.

The enc_key_data.valid had been set to true when key has been added
to the encryption context, but in case when boot_enc_drop was called,
on the same context, the flag remained true, even though the context
may no longer hold any valid context nor key.
The commit moves the enc_key_data invalidation to enc_key_drop.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>