configs/rdn2cfg3: add initial build configuration files

Add the initial build configuration file for RD-N2-Cfg3 platform.
Support for busybox, buildroot, uefi and secure boot is enabled.

Signed-off-by: Tony K Nadackal <tony.nadackal@arm.com>
Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>
Change-Id: Iae9d84131dc15b7604813e67e2e731a08577e02c