siemens: fix upgrade mechanism based on bootcount

Correct location of saveenv and remove not active variable.

Add CONFIG_BOOT_RETRY_TIME and CONFIG_RESET_TO_RETRY to
reboot board in case of empty kernel partition. Without
these defines an empty kernel partition leads to an
abort of boot process and one remains in u-boot prompt.

Signed-off-by: Samuel Egli <samuel.egli@siemens.com>
Cc: Matthias Michel <matthias.michel@siemens.com>
siemens: general cleanup of dxr2, pxm2 and rut boards

all:
 * Remove net boot from bootcmd
   Ping can cause a crash on boards without ethernet phy.
   net_nfs command is used only for development

 * Add reset at the end of bootcmd
   In order to have an immediate reset of the boot when bootcmd
   fails, add reset at the end of bootcmd.

rut:
 * add nand_img_size

dxr2:
 * update nand_img_size

 * ddr3 timings updated with iocontrol property that can be
   modified via eeprom. New default parameters from software
   leveling with draco ES2.

Signed-off-by: Samuel Egli <samuel.egli@siemens.com>
Cc: Pascal Bach <pascal.bach@siemens.com>
Cc: Roger Meier <r.meier@siemens.com>
siemens: add corvus board

Signed-off-by: Boris Schmidt <boris.schmidt@siemens.com>