FVP: restrict dynamic config to Unix build environment
FVP: restrict dynamic config to Unix build environment

This patch restricts building the dynamic config DTBs to the Unix
build environment as the Device Tree compiler may not be available
on other build environments.

Change-Id: Ie690e80010a174300e966240fd977b37561156e0
Signed-off-by: Soby Mathew <4d7af7c5c404e07961f9e1e79d6f17409c1d7db7@arm.com>

achingupta/arm-trusted-firmware,achingupta/arm-trusted-firmware