Makefile: Use CROSS_COMPILE when CROSS_COMPILE_(HOST|TA) not defined

Setting both CROSS_COMPILE_HOST and CROSS_COMPILE_TA can be tedious,
especially when these will most often be the same and many of us have
CROSS_COMPILE defined already for this reason. Fallback to this when
CROSS_COMPILE_HOST or CROSS_COMPILE_TA are not explicitly defined.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: David Brown <david.brown@linaro.org>

Makefile

@@ -16,6 +16,10 @@ else
 	echo := @:
 endif
 
+# If _HOST or _TA specific compilers are not specified, then use CROSS_COMPILE
+CROSS_COMPILE_HOST ?= $(CROSS_COMPILE)
+CROSS_COMPILE_TA ?= $(CROSS_COMPILE)
+
 .PHONY: all
 ifneq ($(wildcard $(TA_DEV_KIT_DIR)/host_include/conf.mk),)
 all: xtest ta