pico-imx6ul: Add a README file

Add a README file to help users to install U-boot binary into the eMMC.

Signed-off-by: Vanessa Maegima <vanessa.maegima@nxp.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>

board/technexion/pico-imx6ul/README

+How to Update U-Boot on Pico-imx6ul board
+-----------------------------------------
+
+Required software on the host PC:
+
+- imx_usb_loader: https://github.com/boundarydevices/imx_usb_loader
+
+- dfu-util: http://dfu-util.sourceforge.net/releases/
+
+Build U-Boot for Pico:
+
+$ make mrproper
+$ make pico-imx6ul_defconfig
+$ make
+
+This will generate the U-Boot binary called u-boot.imx.
+
+Put pico board in USB download mode (refer to the document
+http://www.wandboard.org/images/hobbit/hobbitboard-imx6ul-reva1.pdf page 15)
+
+Connect a USB to serial adapter between the host PC and pico
+
+Connect a USB cable between the OTG pico port and the host PC
+
+Open a terminal program such as minicom
+
+Copy u-boot.imx to the imx_usb_loader folder.
+
+Load u-boot.imx via USB:
+
+$ sudo ./imx_usb u-boot.imx
+
+Then U-Boot should start and its messages will appear in the console program.
+
+Use the default environment variables:
+
+=> env default -f -a
+=> saveenv
+
+Run the DFU command:
+=> dfu 0 mmc 0
+
+Transfer u-boot.imx that will be flashed into the eMMC:
+
+$ sudo dfu-util -D u-boot.imx -a boot
+
+Then on the U-Boot prompt the following message should be seen after a
+successful upgrade:
+
+#DOWNLOAD ... OK
+Ctrl+C to exit ...
+
+Remove power from the pico board.
+
+Put pico board into normal boot mode
+
+Power up the board and the new updated U-Boot should boot from eMMC.