linux-initrd: Delete files from the initrd ramfs when switching roots.

* guix/build/linux-initrd.scm (switch-root): Delete file from the old
  root.  Chdir to / after 'chroot' call.  Re-open file descriptors 0, 1,
  and 2.
  (boot-system): Move 'loading' message after the 'switch-root' call.
* gnu/system.scm (operating-system-boot-script): Add loop that closes
  file descriptor before calling 'execl'.