toradex: imx: enable BOOTCOUNT feature

toradex: imx: enable BOOTCOUNT feature

This introduces automatic boot counter that increases after every
reset.After a power-on reset, it will be initialized with 1,
and each reboot will increment the value by 1. By default it's
disabled if bootlimit isn't set.

To enable this feature you have set bootcount limit ("bootlimit"),
alternate boot action ("altbootcmd") that will be performed if
the new value of bootcount exceeds the value of bootlimit, and
"upgrade_available" to let U-Boot automatically increase and save
the counter value after every reset:

> setenv bootlimit 5
> setenv upgrade_available 1
> setenv altbootcmd "bootm ..."

In case the bootlimit exceeds, the message will be shown and
albootcmd executed:
Warning: Bootlimit (5) exceeded. Using altbootcmd.

To reset bootcount run:
> bootcount reset

Print current value:
> bootcount print

Signed-off-by: Igor Opaniuk <igor.opaniuk@toradex.com>