evm: Extend evm= with x509. allow_metadata_writes and complete values

hulk inclusion
category: feature
feature: IMA Digest Lists extension
bugzilla: 46797

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Introduce three new values for evm= kernel option:

x509: enable EVM by setting x509 flag;
allow_metadata_writes: permit metadata modificatons;
complete: don't allow further changes of the EVM status.

Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Signed-off-by: Tianxing Zhang <zhangtianxing3@huawei.com>
Reviewed-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>