configs/rdn2cfg2: set arm-tf build variant

RD-N2-Cfg2 is a variant of RD-N2 in the TF-A (arm-tf) builds. Set
`ARM_TF_BUILD_VARIANT` flag as rdn2cfg2 so that build-arm-tf.sh script
will consider it as a variant of rdn2 and set TBBR enable flag according
to rdn2cfg2 config.

Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>
Change-Id: I89920bcc939b5851ebc41b6f96ba39837a363d71