ima: ima_calc_boot_agregate must use SHA1

With multiple hash algorithms, ima_hash_tfm is no longer guaranteed to be sha1.
Need to force to use sha1.

Changelog:
- pass ima_digest_data to ima_calc_boot_aggregate() instead of char *
  (Roberto Sassu);
- create an ima_digest_data structure in ima_add_boot_aggregate()
  (Roberto Sassu);
- pass hash->algo to ima_alloc_tfm() (Roberto Sassu, reported by Dmitry).
- "move hash definition in ima_add_boot_aggregate()" commit hunk to here.
- sparse warning fix - Fengguang Wu

Signed-off-by: Dmitry Kasatkin <d.kasatkin@samsung.com>
Signed-off-by: Roberto Sassu <roberto.sassu@polito.it>
Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>