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 <ps.report@gmx.net>
[yann.morin.1998@free.fr: fix case of no config.txt provided]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

configs/raspberrypicm4io_defconfig

@@ -22,6 +22,7 @@ BR2_LINUX_KERNEL_NEEDS_HOST_OPENSSL=y
 
 BR2_PACKAGE_RPI_FIRMWARE=y
 BR2_PACKAGE_RPI_FIRMWARE_VARIANT_PI4=y
+BR2_PACKAGE_RPI_FIRMWARE_CONFIG_FILE="board/raspberrypicm4io-64/config_cm4io.txt"
 
 # Required tools to create the SD image
 BR2_PACKAGE_HOST_DOSFSTOOLS=y
@@ -38,4 +39,3 @@ BR2_TARGET_ROOTFS_EXT2_SIZE="120M"
 # BR2_TARGET_ROOTFS_TAR is not set
 BR2_ROOTFS_POST_BUILD_SCRIPT="board/raspberrypicm4io/post-build.sh"
 BR2_ROOTFS_POST_IMAGE_SCRIPT="board/raspberrypicm4io/post-image.sh"
-BR2_ROOTFS_POST_SCRIPT_ARGS="--add-miniuart-bt-overlay"