imagebuilder: move handling of DEFAULT_PACKAGES into shareable place

It seems, that handling of DEFAULT_PACKAGES is needed in more places, so
lets move it into dedicated include file so it can be easily shared.

Signed-off-by: Petr Štetiar <ynezz@true.cz>
Link: https://github.com/openwrt/openwrt/pull/16986
Signed-off-by: Robert Marko <robimarko@gmail.com>

include/default-packages.mk

@@ -0,0 +1,5 @@
+ifneq ($(CONFIG_USE_APK),)
+  DEFAULT_PACKAGES += apk-mbedtls
+else
+  DEFAULT_PACKAGES += opkg
+endif

target/imagebuilder/files/Makefile

@@ -26,6 +26,7 @@ include $(INCLUDE_DIR)/debug.mk
 include $(INCLUDE_DIR)/depends.mk
 include $(INCLUDE_DIR)/rootfs.mk
 
+include $(INCLUDE_DIR)/default-packages.mk
 include $(INCLUDE_DIR)/version.mk
 export REVISION
 export SOURCE_DATE_EPOCH