Add gbs option to enable the TZ backend support

Add gbs option to enable the TZ backend support

To enable the TZ backend support add following option to the gbs build:
--define "tz_backend ON". If the option is not set or is set to value
different than ON, the TZ backend support is disabled (it is disabled by
default).
When the TZ backend is disabled, some tests (T6* and T7*) are not built.

The same option has been added to the key-manager (branch tizen).

The key-manager-ta requires the following gbs option for these tests to
work properly: --define "test_key ON".

Change-Id: If1c27d8ae556f6882f65c4ace8bb4c1759656893