Support board specific RPi firmware configuration (#1567)

Support optional board specific default RPi firmware configuration file
(config.txt). Also rename from boot-env.txt to config.txt since this
file is not read by the U-Boot boot loader but the Raspberry Pi specific
boot firmware.

buildroot-external/board/raspberrypi/config.txt

@@ -0,0 +1,76 @@
+# For more options and information see
+# http://rpf.io/configtxt
+# Some settings may impact device functionality. See link above for details
+
+# HAOS - don't change it!
+disable_splash=1
+kernel=u-boot.bin
+
+# uncomment for aarch64 bit support
+#arm_64bit=1
+
+# uncomment to enable primary UART console
+#enable_uart=1
+
+# uncomment if you get no picture on HDMI for a default "safe" mode
+#hdmi_safe=1
+
+# uncomment this if your display has a black border of unused pixels visible
+# and your display can output without overscan
+#disable_overscan=1
+
+# uncomment the following to adjust overscan. Use positive numbers if console
+# goes off screen, and negative if there is too much border
+#overscan_left=16
+#overscan_right=16
+#overscan_top=16
+#overscan_bottom=16
+
+# uncomment to force a console size. By default it will be display's size minus
+# overscan.
+#framebuffer_width=1280
+#framebuffer_height=720
+
+# uncomment if hdmi display is not detected and composite is being output
+#hdmi_force_hotplug=1
+
+# uncomment to force a specific HDMI mode (this will force VGA)
+#hdmi_group=1
+#hdmi_mode=1
+
+# uncomment to force a HDMI mode rather than DVI. This can make audio work in
+# DMT (computer monitor) modes
+#hdmi_drive=2
+
+# uncomment to increase signal to HDMI, if you have interference, blanking, or
+# no display
+#config_hdmi_boost=4
+
+# uncomment for composite PAL
+#sdtv_mode=2
+
+# Uncomment to disable continous SD-card poll (for USB SSD)
+#dtparam=sd_poll_once=on
+
+# Uncomment some or all of these to enable the optional hardware interfaces
+#dtparam=i2c_arm=on
+#dtparam=i2s=on
+#dtparam=spi=on
+
+# Uncomment this to enable the lirc-rpi module
+#dtoverlay=lirc-rpi
+
+# Uncomment this to enable GPIO support for RPI-RF-MOD/HM-MOD-RPI-PCB
+#enable_uart=1
+#dtparam=i2c_arm=on
+#dtoverlay=miniuart-bt
+#dtoverlay=rpi-rf-mod
+
+# Additional overlays and parameters are documented /boot/overlays/README
+
+# Enable audio (loads snd_bcm2835)
+dtparam=audio=on
+
+[all]
+#dtoverlay=vc4-fkms-v3d
+#max_framebuffers=2