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>

board/raspberrypi/config_3.txt

@@ -0,0 +1,29 @@
+# Please note that this is only a sample, we recommend you to change it to fit
+# your needs.
+# You should override this file using BR2_PACKAGE_RPI_FIRMWARE_CONFIG_FILE.
+# See http://buildroot.org/manual.html#rootfs-custom
+# and http://elinux.org/RPiconfig for a description of config.txt syntax
+
+start_file=start.elf
+fixup_file=fixup.dat
+
+kernel=zImage
+
+# To use an external initramfs file
+#initramfs rootfs.cpio.gz
+
+# Disable overscan assuming the display supports displaying the full resolution
+# If the text shown on the screen disappears off the edge, comment this out
+disable_overscan=1
+
+# How much memory in MB to assign to the GPU on Pi models having
+# 256, 512 or 1024 MB total memory
+gpu_mem_256=100
+gpu_mem_512=100
+gpu_mem_1024=100
+
+# fixes rpi (3B, 3B+, 3A+, 4B and Zero W) ttyAMA0 serial console
+dtoverlay=miniuart-bt
+
+# enable autoprobing of Bluetooth driver without need of hciattach/btattach
+dtoverlay=krnbt=on