km/common: add support for second flash

Add support for a second flash.
By default, a single flash chip is set up:
- called 'boot'
- mtd name for ubi 'ubi0'
MTDIDS and MTDPARTS may be overwritten to add a second flash.
The 'ubiattach' command is featured in two versions:
- if CONFIG_KM_UBI_PARTITION_NAME_APP is not defined:
  this is the version as up to now
- if CONFIG_KM_UBI_PARTITION_NAME_APP is defined:
  a check on 'actual_bank' will be done to determine the flash to boot
  from.

Use CONFIG_KM_UBI_PARTITION_NAME_BOOT and
CONFIG_KM_UBI_PARTITION_NAME_APP to define where to put the bootloader
and the applications.

Example:
In the board config do:
... snip ...

       "boot:"                                                 \
               "768k(u-boot),"                                 \
               "128k(env),"                                    \
               "128k(envred),"                                 \
               "-(" CONFIG_KM_UBI_PARTITION_NAME_BOOT ");"     \
       "app:"                                                  \
               "-(" CONFIG_KM_UBI_PARTITION_NAME_APP ");"

... snap ...

You may also need to enable the second flash:
... snip ...
/* additional featured for COGE5 */
... snap ...

Signed-off-by: Andreas Huber <andreas.huber@keymile.com>
cc: Kim Phillips <kim.phillips@freescale.com>
cc: Valentin Longchamp <valentin.longchamp@keymile.com>
cc: Gerlando Falauto <gerlando.falauto@keymile.com>
Signed-off-by: Holger Brunck <holger.brunck@keymile.com>
Signed-off-by: Kim Phillips <kim.phillips@freescale.com>