arm/km: adapt bootcounter evaluation

The bootcounter (stored in the RAM) is not enough protected with the 4 Bytes
BOOTCOUNT_MAGIC against bit errors due to short power loss or holding a system
in RESET. It has been seen, that the bootcounter value has been changed due to
a bit flip on a system holding in RESET, but the BOOTCOUNT_MAGIC was still valid.

A bit pattern with 4000 bytes (after BOOTCOUNT_MAGIC) has been implemented,
which should be enough to detect a bit error.

Signed-off-by: Holger Brunck <holger.brunck@keymile.com>
Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
cc: Prafulla Wadaskar <prafulla@marvell.com>