98integrity: support loading x509 into the trusted/builtin .evm keyring

98integrity: support loading x509 into the trusted/builtin .evm keyring

This implements logic analogous to the one already implemented in
ima-keys-load.sh, only for the .evm/_evm keyrings.

If the kernel was built with CONFIG_IMA_TRUSTED_KEYRING then the kernel
initially creates and configures .ima and .evm keyrings. These keyrings
only accept x509 certificates that have been signed by a local CA which
belongs to the kernel builtin trusted keyring.

Thus if such a keyring is already present then additional evm keys
should be loaded into them. If this is not the case then the _evm
keyring needs to be created in userspace and keys will be loaded into
it instead.

Before this change dracut always created the _evm keyring and loaded
keys into it without considering an existing .evm keyring. In case of
CONFIG_IMA_TRUSTED_KEYRING being enabled, the _evm keyring will not be
used by the kernel, however, and EVM digital signatures will not work as
expected.