package/rpi-firmware: rework boot/config file handling

package/rpi-firmware: rework boot/config file handling

Try to be less smart (focused on the one target/one use-case),
instead reduce the rpi-firmware package to a selectable list
of (verbatim) installed firmware files.

- change rpi-firmware config handling from rpi-variant/rpi-flavour
  choices to bootcode.bin, pi-default/-extended/-cut-down and
  pi4-/default/-extended/-cut-down selection

- add BR2_PACKAGE_RPI_FIRMWARE_CONFIG_FILE option to select installable
  config.txt file

- remove config.txt modify code/handling from raspberry post-image.sh
  script

- add different customized config.txt files to the raspberry board
  section

- change dtoverlay krnbt from 'dtoverlay=miniuart-bt,krnbt=on' to extra line
  with explanation comment

- change raspberry defconfigs to select appropiate rpi-firmware
  and config.txt files

- change genimage-raspberrypi4.cfg/genimage-raspberrypi4-64.cfg to
  use start4.elf and fixup4.dat

- update board/raspberrypi/readme.txt (add optional files fixup4.dat,
  start4.elf and zImage)

With this changes a better support for custom use-cases should
be possible, specially multi-target SD cards as suggested by
Stefan Agner ([1]).

[1] http://lists.busybox.net/pipermail/buildroot/2021-February/303318.html

Signed-off-by: Peter Seiderer <[email protected]>
[[email protected]: fix case of no config.txt provided]
Signed-off-by: Yann E. MORIN <[email protected]>