[script] refactor `bootstrap` and `make-raspbian.bash`

[lmnotran]

script/bootstrap.bash

I think the bootstrap script was originally taken from https://github.com/openthread/ot-br-posix/blob/main/tests/scripts/bootstrap.sh.

Most of the script isn't required since the OTBR build occurs in the mounted Raspberry Pi OS chroot, not the host machine. Within the Raspberry Pi OS chroot, the OTBR requirements are already being installed in script/otbr-setup.bash

This PR removes most of the unnecessary packages and also makes the bootstrap script more flexible, allowing smaller portions of the script to be run via command line arguments

Also, the download of RaspiOS has been moved to script/make-raspbian.bash

script/make-raspbian.bash

• cleanup() has been refactored
  • Any running processes from $QEMU_ROOT are killed before unmounting
• /etc/resolv.conf is mounted from the host into $QEMU_ROOT.
  • This is necessary for environments which block public DNS, such as in some corporate networks.
  • The host-machine's DNS configuration is not included in the output raspios image. Instead, the original /etc/resolv.conf from the raspios .img will be present in the output image

[lmnotran]

@jwhui please review when you get a chance