Finish unifying LOCAL_CFLAGS

fixup "Create common.make with LOCAL_CFLAGS and friends"

The code wasn't what I had intended, although it was functionally
equivalent. Make it more readable and more robust.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/scripts/common.make b/scripts/common.make
index 2f27d0e..b115076 100644
--- a/scripts/common.make
+++ b/scripts/common.make
@@ -5,7 +5,7 @@
 WARNING_CXXFLAGS ?= -Wall -Wextra -Wformat=2 -Wno-format-nonliteral
 LDFLAGS ?=
 
-LOCAL_CFLAGS = $(WARNING_CFLAGS) -I../tests/include -I../include -D_FILE_OFFSET_BITS=64
+LOCAL_CFLAGS = $(WARNING_CFLAGS) -I$(MBEDTLS_TEST_PATH)/include -I../include -D_FILE_OFFSET_BITS=64
 LOCAL_CXXFLAGS = $(WARNING_CXXFLAGS) -I../include -I../tests/include -D_FILE_OFFSET_BITS=64
 LOCAL_LDFLAGS = ${MBEDTLS_TEST_OBJS} 		\
 		-L../library			\