Improved pmbootstrap init

Created by: ollieparanoid

• Check if it's possible to create and read from a device node directly when initializing a chroot (closes #472 (closed))
• Copy the Qemu binary into the forign-arch chroots before initializing them, so the post-install scripts directly work during the chroot setup and we don't need to call apk fix afterwards
• Use pmb.helpers.repo.update(), which only updates the APKINDEX files if they are older than 4 hours, instead of using apk's repo update function which always downloads the APKINDEX files
  • Chroot initialization
  • Getting the initial APKINDEX to download apk-tools-static
  • Updating the APKINDEX at the start of pmbootstrap install
• Fixed a bug in from_chroot_suffix: the buildroot_x86_64 has architecture x86_64, not x86. This was randomly discovered because Travis didn't have the APKINDEX files in buildroot_x86_64, because they didn't get mounted right.

All in all this PR will make the workflow a bit faster and pmbootstrap will produce less traffic.