board/rpi3: Enable UART console

U-Boot will not boot without the primary UART console enabled; the Pi
just hangs at the rainbow splash screen.  There are three ways to enable
a UART console, but disabling Bluetooth is usually the best, as the
others have performance implications.  Applications that need Bluetooth
will have to choose one of the other options, depending on their
specific requirements.

board/raspberrypi3/config.txt

@@ -4,3 +4,9 @@ fixup_file=fixup.dat
 kernel=u-boot.bin
 
 disable_overscan=1
+
+# U-Boot requires a UART port.  There are three options for enabling one:
+# 1. Disable Bluetooth with dtoverlay=disable-bt
+# 2. Swap Bluetooth to the mini UART with dtoverlay=miniuart-bt
+# 3. Enable the mini UART as primary with enable_uart=1
+dtoverlay=disable-bt