doc: booting with U-Boot from SD card

documentation.txt

@@ -28,7 +28,7 @@ the NAND or bootloader happens.
 
 
 Use cases
-~~~~~~~~
+~~~~~~~~~
 In fact, most users do not need it. Either way, possible use cases:
 
 -Changing typos in bootscripts which prevent an operation system
@@ -48,11 +48,12 @@ In fact, most users do not need it. Either way, possible use cases:
 
 -and more...
 
-Booting rescueOS
-~~~~~~~~~~~~~~~
+
+Booting rescueOS with flasher
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 Note: You can not store persistent data in /. Of
 course, it becomes possible on the mount points you mount the EMMC or sd card
-to. 
+to.
 
 Note: It comes without any promises and without any warranty. Therefore you
 are doing everything at your own risk. 
@@ -93,6 +94,34 @@ should block and wait.  Now connect the N900 to the PC with the data cable; the
 flasher should detect the device, upload rescueOS, launch it, and then exit.
 
 
+Booting rescueOS with U-Boot from SD card
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Use this method if you have installed U-Boot (https://talk.maemo.org/showthread.php?t=81613)
+and battery is so depleted, that flashing doesn't work.
+
+You'll need "mkimage" command from "u-boot-tools" package.
+
+Preparing required files:
+
+mkimage -A arm -O linux -T kernel -C none -a 80008000 -e 80008000 -n kernel -d rescueOS_n900_kernel_1.3.zImage rescueOS_n900_kernel_1.3.uImage
+mkimage -A arm -O linux -T ramdisk -C none -a 0 -e 0 -n initrd -d rescueOS-1.3.img rescueOS-1.3.uimg
+cat >boot.cmd <<EOF
+setenv bootargs rootdelay root=/dev/ram0;
+setenv setup_omap_atag 1;
+setenv mmckernfile rescueOS_n900_kernel_1.3.uImage;
+setenv mmcinitrdfile rescueOS-1.3.uimg;
+setenv mmcscriptfile;
+run trymmckerninitrdboot;
+EOF
+mkimage -c none -A arm -T script -d boot.cmd boot.scr
+
+Preparing SD card:
+Your SD card needs to have the first partition formatted as ext2/3/4 or FAT.
+Put generated files (without boot.cmd) in the root directory.
+
+Now you should be able to boot rescueOS with generic "External SD card" option from U-Boot.
+
+
 ~~~~~~~~~~~~~~~
 How to use it
 ~~~~~~~~~~~~~~~
@@ -115,7 +144,7 @@ code_reset
 # This will set the lock code back to the default ('12345').
 
 USB Networking
--------------
+--------------
 /rescueOS/usbnetworking-enable.sh
 Sets up USB networking in the following way:
 ifconfig usb0 192.168.2.15 up