olimex_a20_olinuxino_*: remove old SD writing instructions

Now that all the boards have been converted to use genimage, the old
instructions on how to partition and write an SD card are useless.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

board/olimex/a20_olinuxino/readme.txt

@@ -61,8 +61,8 @@ After building, you should get a tree like this:
     `-- zImage
 
 
-How to write the SD card - boards with sdcard.img
-=================================================
+How to write the SD card
+========================
 
 The sdcard.img file is a complete bootable image ready to be written
 on the boot medium. To install it, simply copy the image to a uSD
@@ -72,59 +72,5 @@ card:
 
 Where 'sdX' is the device node of the uSD.
 
-
-How to write the SD card - other boards
-=======================================
-
-
-Prepare the SD card
--------------------
-
-Erase existing stuff, and create an unique Linux partition with `fdisk`.
-
-    # fdisk /dev/sdX
-    Command (m for help): o
-    Building a new DOS disklabel with disk identifier 0xf9e1616a.
-    Changes will remain in memory only, until you decide to write them.
-    After that, of course, the previous content won't be recoverable.
-
-    Command (m for help): n
-    Partition type:
-        p   primary (0 primary, 0 extended, 4 free)
-        e   extended
-    Select (default p): p
-    Partition number (1-4, default 1): 1
-    First sector (2048-7626751, default 2048): 2048
-    Last sector, +sectors or +size{K,M,G} (2048-7626751, default 7626751): 
-    Using default value 7626751
-
-    Command (m for help): p
-
-    Disk /dev/sdX: 3904 MB, 3904897024 bytes
-    4 heads, 16 sectors/track, 119168 cylinders, total 7626752 sectors
-    Units = sectors of 1 * 512 = 512 bytes
-    Sector size (logical/physical): 512 bytes / 512 bytes
-    I/O size (minimum/optimal): 512 bytes / 512 bytes
-    Disk identifier: 0xf9e1616a
-
-            Device Boot      Start         End      Blocks   Id  System
-    /dev/sdX1                 2048     7626751     3812352   83  Linux
-
-    Command (m for help): w
-    The partition table has been altered!
-
-    Calling ioctl() to re-read partition table.
-    Syncing disks.
-
-Copy images on the SD card
---------------------------
-
-    # dd if=output/images/rootfs.ext2 of=/dev/sdX1
-    # dd if=output/images/u-boot-sunxi-with-spl.bin of=/dev/sdX bs=1024 seek=8
-
-
-Finish
-======
-
 Eject the SD card, insert it in the A20-OLinuXino board, and power it up.