install: handle chroot unmount/mount properly (MR 2252)

We used to called pmb.chroot.shutdown() with "only_install_related=True"
here to unmount everything in the chroot so we could reliably "df" it to
figure out the rootfs image size.

Since ("pmb.chroot: only init once"), the chroot is no longer
automatically spun back up when we run mkinitfs, and the in-pmbootstrap
marker isn't recreated.

We only actually need to unmount the chroot anyway, so let's restrict it
to just that and remount things again afterwards.

This avoids weird side effects (like the native chroot being
shutdown?!), but this is something we should look into more in the
future since we're still doing a finnicky balancing act to manage chroot
state here.

Signed-off-by: Caleb Connolly <caleb@postmarketos.org>

pmb/chroot/init.py

@@ -55,20 +55,6 @@ def mark_in_chroot(chroot: Chroot=Chroot.native()):
         pmb.helpers.run.root(["touch", in_chroot_file])
 
 
-def setup_qemu_emulation(chroot: Chroot):
-    arch = chroot.arch
-    if not arch.cpu_emulation_required():
-        return
-
-    arch_qemu = arch.qemu()
-
-    # mount --bind the qemu-user binary
-    pmb.chroot.binfmt.register(arch)
-    pmb.helpers.mount.bind_file(Chroot.native() / f"/usr/bin/qemu-{arch_qemu}",
-                                chroot / f"usr/bin/qemu-{arch_qemu}-static",
-                                create_folders=True)
-
-
 def init_keys():
     """
     All Alpine and postmarketOS repository keys are shipped with pmbootstrap.
@@ -131,7 +117,6 @@ def init(chroot: Chroot=Chroot.native(), usr_merge=UsrMerge.AUTO,
         return
 
     pmb.chroot.mount(chroot)
-    setup_qemu_emulation(chroot)
     mark_in_chroot(chroot)
     if (chroot / "bin/sh").is_symlink():
         pmb.config.workdir.chroot_check_channel(chroot)