pcrphase: rename binary to pcrextend

pcrphase: rename binary to pcrextend

The tool initially just measured the boot phase, but was subsequently
extended to measure file system and machine IDs, too. At AllSystemsGo
there were request to add more, and make the tool generically
accessible.

Hence, let's rename the binary (but not the pcrphase services), to make
clear the tool is not just measureing the boot phase, but a lot of other
things too.

The tool is located in /usr/lib/ and still relatively new, hence let's
just rename the binary and be done with it, while keeping the unit names
stable.

While we are at it, also move the tool out of src/boot/ and into its own
src/pcrextend/ dir, since it's not really doing boot related stuff
anymore.