Use abspath to dereference $BUILD_BASE

If the user tries to change BUILD_BASE to put the build products outside
the build tree the compile will fail due to hard coded assumptions that
$BUILD_BASE is a relative path. Fix by using $(abspath $(BUILD_BASE))
to rationalize to an absolute path every time and remove the relative
path assumptions.

This patch also adds documentation that BUILD_BASE can be specified by
the user.

Signed-off-by: Grant Likely <grant.likely@arm.com>
Signed-off-by: Manish Pandey <manish.pandey2@arm.com>
Change-Id: Ib1af874de658484aaffc672f30029b852d2489c8
diff --git a/plat/nvidia/tegra/platform.mk b/plat/nvidia/tegra/platform.mk
index 3d61f06..a4724e6 100644
--- a/plat/nvidia/tegra/platform.mk
+++ b/plat/nvidia/tegra/platform.mk
@@ -62,7 +62,7 @@
 $(eval $(call add_define,RELOCATE_BL32_IMAGE))
 
 # modify BUILD_PLAT to point to SoC specific build directory
-BUILD_PLAT	:=	${BUILD_BASE}/${PLAT}/${TARGET_SOC}/${BUILD_TYPE}
+BUILD_PLAT	:=	$(abspath ${BUILD_BASE})/${PLAT}/${TARGET_SOC}/${BUILD_TYPE}
 
 # platform cflags (enable signed comparisons, disable stdlib)
 TF_CFLAGS	+= -Wsign-compare -nostdlib