Since F2FS is the default filesystem, f2fs-tools is a must have before build

Documents issues#244, which was
caused by ancient build files being reused.

Clarify that the debos in Debian Buster won't work.

Related: #4

Converting the rootfs inside the recipe takes a long time, as it 
requires executing target achitecture binaries through qemu.

In order to speed up the process, we can simply do it in the build 
script.

Fixes issues#64

* build.sh: Add amd64 (UEFI) and amd64-legacy (BIOS) device
* image-amd64.yaml: Based on image-sunxi.yaml. Pinephone hardware
  specifics removed. Added amd64 linux kernel and grub bootloader. Also
  switched to gpt partitioning scheme
* image-amd64-legacy.yaml: Same as image-amd64.yaml except it still uses
  msdos partitioning scheme
* README.md: Briefly describe how to build and run x86_64 qemu images