Precomputation

PCR 8 (First sector of TrustedGRUB2 kernel, i.e. diskboot.img)

First compute SHA1 hash of the diskboot.img. It's the first sector after the MBR, i.e. sector 2. Replace 'sdX' with your actual device:

dd if=/dev/sdX bs=512 skip=1 count=1 | sha1sum

PCR8 is then:

PCR8 = sha1sum( 0000000000000000000000000000000000000000 || SHA1 from command above )

For example this command chain could be used to calculate this:

echo -n 000000000000000000000000000000000000000076cd3a3f9d64bdf528054aae025b36b9235d99be | xxd -r -p | sha1sum -b | awk '{print $1}'