Framework submodule: fix the libtestdriver1 build

`make -C tests libtestdriver1` copies `library/Makefile` to
`tests/libtestdriver1/library/Makefile`, where `../framework` does not point
to the framework submodule.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/library/Makefile b/library/Makefile
index d5e4f64..67f91f6 100644
--- a/library/Makefile
+++ b/library/Makefile
@@ -1,4 +1,8 @@
-include ../framework/exported.make
+ifndef MBEDTLS_PATH
+MBEDTLS_PATH := ..
+endif
+
+include $(MBEDTLS_PATH)/framework/exported.make
 
 # Also see "include/mbedtls/mbedtls_config.h"
 
diff --git a/tests/Makefile b/tests/Makefile
index f82c267..c2a0b84 100644
--- a/tests/Makefile
+++ b/tests/Makefile
@@ -208,6 +208,7 @@
 	s/\b(?=mbedtls_|psa_)/libtestdriver1_/g;
 endef
 
+libtestdriver1.a: export MBEDTLS_PATH := $(patsubst ../..//%,/%,../../$(MBEDTLS_PATH))
 libtestdriver1.a:
 	# Copy the library and fake a 3rdparty Makefile include.
 	rm -Rf ./libtestdriver1