Check arguments when building manual tests

Manual tests require setting USE_NVM=1 and NEW_TEST_SESSION=0
otherwise they will not function properly (as test results will
not be preserved across reboots). We should check that these
variables have the required values during the build.

Change-Id: I72fc8a4badf107fe2ebd6bfb88631828e14a77b3
Signed-off-by: Paul Beesley <paul.beesley@arm.com>