Measure empty PK and KEK EFI vars

Measure empty PK and KEK EFI vars

The OVMF UEFI firmware is measuring PK and KEK when secure boot is
disabled, and those variables are absent.  This can be checked via the
event log to see that there are extensions for PCR 7 associated with PK
and KEK events of type EV_EFI_VARIABLE_DRIVER_CONFIG.

When running the "lock-secureboot-policy" verb, pcrlock complains that
those variables are not found and refuse to generate the
240-secureboot-policy.pcrlock.d/generated.pcrlock file.

The "TCG PC Client Platform Firmware Profile Specification Version 1.05
Revision 23"[1] from May 7, 2021, in section "3.3.4.8 PCR[7] - Secure
Boot Policy Measurements", point 10.b:

If reading a UEFI variable returns UEFI_NOT_FOUND, platform firmware
SHALL measure the absence of the variable. The
UEFI_VARIABLE_DATA.VariableDataLength field MUST be set to zero and
UEFI_VARIABLE_DATA.VariableData field will have a size of zero.

This patch mark those variables to be marked as "synthesize empty",
generating the correct hash for those variables.

Signed-off-by: Alberto Planas <aplanas@suse.com>