gen_init_cpio: add support for file metadata

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

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This patch adds support for file metadata (only TYPE_XATTR metadata type).
gen_init_cpio has been modified to read xattrs from files that will be
added to the image and to include file metadata as separate files with the
special name 'METADATA!!!'.

This behavior can be selected by setting the desired file metadata type as
value for CONFIG_INITRAMFS_FILE_METADATA.

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>