TA dev kit: Clang support

Updates ta/mk/ta_dev_kit.mk and other makefiles so that the COMPILER
variable can be used when building TAs: make COMPILER=clang ...

Signed-off-by: Jerome Forissier <jerome@forissier.org>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>
diff --git a/Makefile b/Makefile
index acc0edf..a41218c 100644
--- a/Makefile
+++ b/Makefile
@@ -69,6 +69,7 @@
 endif
 endif
 
+SCRIPTS_DIR := scripts
 
 include core/core.mk