Add U-Boot Compile instruction

docs/uboot.md

@@ -1,2 +1,31 @@
-!!! info
+## Cross compiler
-    To be done.
+
+Under Debian / Ubuntu you need first to install the necessary packages and tools for cross compiling for ARM.
+
+```bash
+sudo apt-get install gcc make gcc-arm-linux-gnueabi binutils-arm-linux-gnueabi
+```
+
+Other option, use Linaro cross compiler 4.9.4 arm-gnueabi toolchain. Download [here](https://releases.linaro.org/components/toolchain/binaries/4.9-2017.01/arm-linux-gnueabi/).
+
+## Compile U Boot
+
+```bash
+git clone https://github.com/helios-4/u-boot-marvell.git
+
+cd u-boot-marvell
+
+export ARCH=arm
+export CROSS_COMPILE="/path/to/cross/compiler"
+export CROSS_COMPILE_BH=${CROSS_COMPILE}
+
+make mrproper
+
+./build.pl -f spi -b armada_38x_helios4
+```
+
+!!! note
+    To compile using Ubuntu 16.04 cross compiler, please apply [this patch](https://github.com/armbian/build/blob/master/patch/u-boot/u-boot-mvebu/tools-bin_hdr-compiler-fixes.patch) to U-Boot source code.
+
+!!! note
+    If failed to compile using Linaro cross compiler 4.9.4 arm-gnueabihf (error about hard float VFP), use **soft-float compiler**