Nokia RX-51: Document debugging options and compile command

Add example command how to compile U-Boot and add new documentation section
describing how to enable early debug UART and verbose log output for N900.

Signed-off-by: Pali Rohár <pali@kernel.org>
diff --git a/doc/board/nokia/rx51.rst b/doc/board/nokia/rx51.rst
index 7c6647b..93502e2 100644
--- a/doc/board/nokia/rx51.rst
+++ b/doc/board/nokia/rx51.rst
@@ -9,6 +9,10 @@
 already configured the board. It is only needed to enable the internal eMMC
 memory via the twl4030 regulator which is not enabled by NOLO.
 
+.. code-block:: bash
+
+   make CROSS_COMPILE=arm-linux-gnueabi- nokia_rx51_defconfig u-boot.bin
+
 NOLO is expecting a kernel image and will treat any image it finds in
 OneNAND as such. This u-boot is intended to be flashed to the N900 like
 a kernel. In order to transparently boot the original kernel, it will be
@@ -160,6 +164,34 @@
     CONFIG_CMD_UBIFS=y
     CONFIG_MTD_UBI_BEB_LIMIT=10
 
+Early output
+------------
+
+Early U-Boot output can be enabled on serial console by adding following lines
+into file ``configs/nokia_rx51_defconfig``::
+
+    CONFIG_DEBUG_UART=y
+    CONFIG_DEBUG_UART_OMAP=y
+    CONFIG_DEBUG_UART_SHIFT=2
+    CONFIG_DEBUG_UART_BASE=0x49020000
+    CONFIG_DEBUG_UART_CLOCK=48000000
+
+Note that early output is not available on USB tty console.
+
+Verbose debug output
+--------------------
+
+Verbose debug output with maximal log level can be enabled by adding following
+lines into file ``configs/nokia_rx51_defconfig``::
+
+    CONFIG_DM_DEBUG=y
+    CONFIG_LOG=y
+    CONFIG_LOGLEVEL=9
+    CONFIG_LOG_MAX_LEVEL=9
+    CONFIG_LOG_DEFAULT_LEVEL=9
+
+And compiling U-Boot by additional make parameter ``KCPPFLAGS=-DLOG_DEBUG``.
+
 Run in QEMU
 -----------