Add support for portable EVM format

Add a --portable argument that generates EVM signatures without using
the inode number and generation or fs UUID.

Signed-off-by: Matthew Garrett <mjg59@google.com>
Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>

Changelog:
- immutable and portable flags are exclusive, prevent enabling both.

src/imaevm.h

@@ -82,6 +82,7 @@ enum evm_ima_xattr_type {
 	EVM_XATTR_HMAC,
 	EVM_IMA_XATTR_DIGSIG,
 	IMA_XATTR_DIGEST_NG,
+	EVM_XATTR_PORTABLE_DIGSIG,
 };
 
 struct h_misc {