measure: add new tool to precalculate PCR values for a kernel image

measure: add new tool to precalculate PCR values for a kernel image

For now, this simply outputs the PCR hash values expected for a kernel
image, if it's measured like sd-stub would do it.

(Later on, we can extend the tool, to optionally sign these
pre-calculated measurements, in order to implement signed PCR policies
for disk encryption.)