config/rdfremontcfg1: add missing TF_A_BUILD_VARIANT flag

TF_A_BUILD_VARIANT will be used by TF-A build-scripts to identify if a
particular platform has a variant or not.

Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>
Change-Id: I6a1e0fe0b1f56b9e9002c94905f31c2008a00218