Add support for ppc64le and add repo_bootstrap support

There is a cyclic dependency between postmarketos-base and postmarketos-initramfs. We can use repo_bootstrap to work around this when bootstrapping a new architecture.

This also adds support for ppc64le and a QEMU target for ppc64le.

Depends on pmbootstrap!2476 (merged).

How to test (on this branch):

$ pmbootstrap init # Select qemu-ppc64le
$ pmbootstrap config mirrors.pmaports none
$ pmbootstrap repo_bootstrap --arch ppc64le pmaports
$ pmbootstrap install
$ pmbootstrap qemu

Fixes #3236 (closed)