efi/libstub: measure loaded initrd info into the TPM

efi/libstub: measure loaded initrd info into the TPM

In an effort to ensure the initrd observed and used by the OS is
the same one that was meant to be loaded, which is difficult to
guarantee otherwise, let's measure the initrd if the EFI stub and
specifically the newly introduced LOAD_FILE2 protocol was used.

Modify the initrd loading sequence so that the contents of the initrd
are measured into PCR9.  Note that the patch is currently using
EV_EVENT_TAG to create the eventlog entry instead of EV_IPL.  According
to the TCP PC Client specification this is used for PCRs defined for OS
and application usage.

Co-developed-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Link: https://lore.kernel.org/r/20211119114745.1560453-5-ilias.apalodimas@linaro.org
[ardb: add braces to initializer of tagged_event_data]
Link: https://github.com/ClangBuiltLinux/linux/issues/1547
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>