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>

tests/Makefile

diff --git a/tests/Makefile b/tests/Makefile
index 7a10af2..848d3c4 100644
--- a/tests/Makefile
+++ b/tests/Makefile
@@ -1,3 +1,4 @@
+MBEDTLS_TEST_PATH = .
 include ../scripts/common.make
 
 # Set this to -v to see the details of failing test cases
@@ -109,7 +110,6 @@
 
 all: $(BINARIES)
 
-MBEDTLS_TEST_PATH = .
 MBEDTLS_TEST_OBJS = $(patsubst %.c,%.o,$(wildcard ${MBEDTLS_TEST_PATH}/src/*.c ${MBEDTLS_TEST_PATH}/src/drivers/*.c))
 MBEDTLS_TEST_OBJS += $(patsubst %.c,%.o,$(wildcard ${MBEDTLS_TEST_PATH}/src/test_helpers/*.c))