Add new package - bcm-fscrypt-key-migration

Mattias Barthelrequested to merge
mattias_#8596_65 into release-6.5

This preinit script package will migrate fsencrypted overlay partition if the new key, key_dev_specific_512_grypt_salt, exists in DTS. From the old key, key_dev_specific_512, to the new key.

(cherry picked from commit eab64a95)

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