ima: Add methods for parsing ima policy configuration string

hulk inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I49KW1


CVE: NA

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IMA subsystem is configured at boot time using kernel command-line
parameters, e.g.: ima_policy=tcb|appraise_tcb|secure_boot. The same
configuration options should be available for the new ima namespace.
Add new functions to parse configuration string and store parsed data
in the new policy data structures. Don't implement it yet, just add the
dummy interface.

Signed-off-by: Krzysztof Struczynski <krzysztof.struczynski@huawei.com>
Reviewed-by: Zhang Tianxing <zhangtianxing3@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>