Changed kernel and initrd compression from gzip to XZ.
Build scripts and help documents are all updated to reflect this change.
This makes the compressed kernel+initrd image 790KiB smaller, although the memory required  for decompression increases from 0.5MiB to 5MiB. (For old hardware, you'll still need 16MiB of memory to load CPUstress either way.)
e --ultra-brute switch in upx to re-compress linpack and mprime binaries. Also compress sensors, stress, stresscpu2, and systester-cli binaries.
This saves 0.97MiB in uncompressed initrd (699KiB when compressed)
pack_buildtxz.sh: exclude initrd.xz instead.
Fix a typo in README.md
Change kernel configs so that useless components are removed.