mx6sabresd: Add README file

Add a README to explain the steps for booting mx6sabresd in different ways:
    1. Booting via Normal U-Boot (u-boot.imx)
    2. Booting via SPL (SPL and u-boot.img)
    3. Booting via Falcon mode (SPL launches the kernel directly)

Signed-off-by: Diego Dorta <diego.dorta@nxp.com>

board/freescale/mx6sabresd/README

+How to use and build U-Boot on mx6sabresd:
+----------------------------------
+
+Currently there are three methods for booting mx6sabresd boards:
+
+1. Booting via Normal U-Boot (u-boot.imx)
+
+2. Booting via SPL (SPL and u-boot.img)
+
+3. Booting via Falcon mode (SPL launches the kernel directly)
+
+
+1. Booting via Normal U-Boot
+----------------------------
+
+$ make mx6qsabresd_defconfig (If you want to build for mx6qsabresd)
+
+or
+
+$ make mx6dlsabresd_defconfig (If you want to build for mx6dlsabresd)
+
+$ make
+
+This will generate the image called u-boot.imx.
+
+- Flash the u-boot.imx binary into the SD card:
+
+$ sudo dd if=u-boot.imx of=/dev/sdb bs=1K seek=1 && sync
+
+
+2. Booting via SPL
+------------------
+
+Other method for building U-Boot on mx6qsabresd and mx6qpsabresd is
+through SPL. In order to do so:
+
+$ make mx6sabresd_spl_defconfig
+$ make
+
+This will generate the SPL image called SPL and the u-boot.img.
+
+- Flash the SPL image into the SD card:
+
+$ sudo dd if=SPL of=/dev/sdb bs=1K seek=1 && sync
+
+- Flash the u-boot.img image into the SD card:
+
+$ sudo dd if=u-boot.img of=/dev/sdbbs=1K seek=69 && sync
+
+
+3. Booting via Falcon mode
+--------------------------
+
+$ make mx6sabresd_spl_defconfig
+$ make
+
+This will generate the SPL image called SPL and the u-boot.img.
+
+- Flash the SPL image into the SD card:
+
+$ sudo dd if=SPL of=/dev/sdb bs=1K seek=1 oflag=sync status=none && sync
+
+- Flash the u-boot.img image into the SD card:
+
+$ sudo dd if=u-boot.img of=/dev/sdbbs=1K seek=69 oflag=sync status=none && sync
+
+Create a partition for root file system and extract it there:
+
+$ sudo tar xvf rootfs.tar.gz -C /media/root
+
+The SD card must have enough space for raw "args" and "kernel".
+To configure Falcon mode for the first time, on U-Boot do the following commands:
+
+- Setup the IP server:
+
+# setenv serverip <server_ip_address>
+
+- Download dtb file:
+
+# dhcp ${fdt_addr} imx6q-sabresd.dtb
+
+- Download kernel image:
+
+# dhcp ${loadaddr} uImage
+
+- Write kernel at 2MB offset:
+
+# mmc write ${loadaddr} 0x1000 0x4000
+
+- Setup kernel bootargs:
+
+# setenv bootargs "console=ttymxc0,115200 root=/dev/mmcblk1p1 rootfstype=ext4 rootwait quiet rw"
+
+- Prepare args:
+
+# spl export fdt ${loadaddr} - ${fdt_addr}
+
+- Write args 1MB data (0x800 sectors) to 1MB offset (0x800 sectors)
+
+# mmc write 18000000 0x800 0x800
+
+- Press KEY_VOL_UP key, power up the board and then SPL binary will
+launch the kernel directly.