configs/rdn2cfg2: add initial build configuration files

Add the initial build configuration files for RD-N2-Cfg2 platform. It
is a multichip variant of RD-N2. Support for uefi, busybox and buildroot
boot are enabled.

Signed-off-by: Aditya Angadi <aditya.angadi@arm.com>
Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>
Change-Id: I61ba1975c22e2bc2cb4aea19349f133b938d16ba