[Android] Add F-Droid latest file (#4869)

This can be added to the build server as an artifact and used for automatic F-Droid updates. Cf. https://github.com/koreader/koreader/issues/2040#issuecomment-474925297 pointed out by @pazos.

.gitignore

@@ -41,6 +41,7 @@ i18n
 
 koreader-android-arm-linux-androideabi*
 koreader-android-i686-linux-android*
+koreader-android-fdroid-latest
 koreader-cervantes-arm-linux-gnueabi*
 koreader-cervantes-arm-cervantes-linux-gnueabi*
 koreader-debian-i686-linux-gnu*

Makefile

@@ -67,6 +67,9 @@ all: $(if $(ANDROID),,$(KOR_BASE)/$(OUTPUT_DIR)/luajit)
 	$(MAKE) -C $(KOR_BASE)
 	install -d $(INSTALL_DIR)/koreader
 	rm -f $(INSTALL_DIR)/koreader/git-rev; echo "$(VERSION)" > $(INSTALL_DIR)/koreader/git-rev
+ifdef ANDROID
+	rm -f android-fdroid-version; echo -e "$(ANDROID_NAME)\n$(ANDROID_VERSION)" > koreader-android-fdroid-latest
+endif
 ifneq ($(or $(EMULATE_READER),$(WIN32)),)
 	cp -f $(KOR_BASE)/ev_replay.py $(INSTALL_DIR)/koreader/
 	@echo "[*] create symlink instead of copying files in development mode"