mbed_crypto: Always describe the current version

Even with a shallow clone of the repo where there are no tags available
to version with, don't error and instead show a unique abbreviated
commit hash as fallback.
diff --git a/scripts/mbed_crypto.make b/scripts/mbed_crypto.make
index 5da5708..c0e5a05 100644
--- a/scripts/mbed_crypto.make
+++ b/scripts/mbed_crypto.make
@@ -213,7 +213,7 @@
 	$(call rename_mbedcrypto,$@)
 
 crypto/VERSION.txt: FORCE
-	@git describe --tags --abbrev=12 --dirty > $@
+	@git describe --tags --abbrev=12 --dirty --always > $@
 
 mbedcrypto.tar.gz: $(LIB_FILES) $(INC_FILES) $(TEST_FILES) $(OTHER_FILES)
 	@echo $@