Vendor: Omni: Allow custom keys

  * This change allow custom keys for rom signature generated with: 

```
  subject='/C=FR/O=Android/OU=Android/CN=Android/emailAddress=android@me.org' 
    for x in testkey releasekey platform shared media; 
    do ./development/tools/make_key ~/.android-certs/$x "$subject"; 
    done 
```

  * Set ROM_KEYS_PATH env var to the folder containing keys.

Change-Id: Ia5a420fc3ff7120b88a5ed7413f78539b9d38629
diff --git a/build/core/certs.mk b/build/core/certs.mk
new file mode 100644
index 0000000..f00e275
--- /dev/null
+++ b/build/core/certs.mk
@@ -0,0 +1,5 @@
+# Allow custom build keys
+ifneq (${ROM_KEYS_PATH},)
+    PRODUCT_DEFAULT_DEV_CERTIFICATE := ${ROM_KEYS_PATH}/releasekey
+    PRODUCT_EXTRA_RECOVERY_KEYS := $(PRODUCT_DEFAULT_DEV_CERTIFICATE)
+endif