trusty: keymint: explicitly set the app name.

Prevent app name being inferred from the directory name and resulting in
app.elf etc...

Bug: 322328911
Test: build.py, inspect build-root
Change-Id: I12d6a9d10a673709bf8d9b2e39ac83fd744a2958
diff --git a/app/rules.mk b/app/rules.mk
index e74e93a..ce6ce93 100644
--- a/app/rules.mk
+++ b/app/rules.mk
@@ -17,6 +17,8 @@
 
 MODULE := $(LOCAL_DIR)
 
+TRUSTY_APP_NAME := keymint
+
 MANIFEST := $(LOCAL_DIR)/manifest.json
 
 MODULE_SRCS += \