cert_create: add non-volatile counter support

This patch adds non-volatile counter support to the Certificate
Generation tool. The TBBR Chain of Trust definition in the tool
has been extended to include the counters as certificate extensions.
The counter values can be specified in the command line.

The following default counter values are specified in the build
system:

  * Trusted FW Non-Volatile counter = 0
  * Non-Trusted FW Non-Volatile counter = 0

These values can be overridden by the platform at build time.

Change-Id: I7ea10ee78d72748d181df4ee78a7169b3ef2720c