configs/rdn2: define new build flags for rdn2

Firmware first error handling is implemented at various software levels.
Define logical build flags to enable or disable the software present at
these levels. Introduce new control flag to enable or disable firmware
first feature. By default the firmware first feature is disabled.

Signed-off-by: Omkar Anand Kulkarni <omkar.kulkarni@arm.com>
Change-Id: I83013da34a54c99cf3b7f95ace4b4c8b6ceccebc