Tegra: modify 'BUILD_PLAT' to point to soc specific build dirs

This patch modifies the 'BUILD_PLAT' makefile variable to point to the soc
specific build directory in order to allow each Tegra soc to have its own
build directory. This way we can keep the build outputs separate and can
keep multiple soc specific builds alive at the same time.
Signed-off-by: Varun Wadekar <vwadekar@nvidia.com>

plat/nvidia/tegra/platform.mk

@@ -32,3 +32,6 @@ SOC_DIR		:=	plat/nvidia/tegra/soc/${TARGET_SOC}
 
 include plat/nvidia/tegra/common/tegra_common.mk
 include ${SOC_DIR}/platform_${TARGET_SOC}.mk
+
+# modify BUILD_PLAT to point to SoC specific build directory
+BUILD_PLAT	:=	${BUILD_BASE}/${PLAT}/${TARGET_SOC}/${BUILD_TYPE}