Implemented an good way to replace base files

banner/Makefile

+include $(TOPDIR)/rules.mk
+
+PKG_NAME:=ffnw-banner
+PKG_VERSION:=1
+PKG_RELEASE:=1
+
+PKG_BUILD_DIR := $(BUILD_DIR)/$(PKG_NAME)
+
+include $(INCLUDE_DIR)/package.mk
+
+define Package/ffnw-banner
+  SECTION:=config
+  CATEGORY:=Freifunk Nordwest
+  TITLE:=Nordwest freifunk shell banner
+  MAINTAINER:=Freifunk Nordwest <tarek@ring0.de>
+  URL:=https://git.nordwest.freifunk.net/ffnw/packages
+  SOURCE:=git@git.nordwest.freifunk.net:ffnw/packages.git
+endef
+
+define Package/ffnw-banner/description
+  Nordwest freifunk shell banner
+endef
+
+define Build/Prepare
+  mkdir -p $(PKG_BUILD_DIR)
+endef
+
+define Build/Configure
+endef
+
+define Build/Compile
+endef
+
+define Package/ffnw-banner/preinst
+  #!/bin/sh
+  cd "$${IPKG_INSTROOT}/etc/"
+  if [ -h "./banner" ] ; then
+    /bin/rm "./banner"
+  elif [ -f "./banner" ] ; then
+    /bin/mv "./banner" "/tmp/banner.openwrt"
+  fi
+  /bin/ln -s "/tmp/banner.openwrt" "/etc/banner"
+  exit 0
+endef
+
+define Package/ffnw-banner/postinst
+  #!/bin/sh
+  cd "$${IPKG_INSTROOT}/etc/"
+  [ -h "./banner" ] && /bin/rm -f "./banner"
+  /bin/mv "./banner.ffnw" "./banner"
+  exit $$?
+endef
+
+define Package/ffnw-banner/prerm
+  #!/bin/sh
+  cd "$${IPKG_INSTROOT}/etc/"
+  if [ -h "./banner" ] ; then
+    [[ "$$(readlink -n ./banner)" == "./banner.ffnw" ]] && \
+    /bin/rm -f "./banner" && \
+    [ -f "/tmp/banner.openwrt" ] && \
+    /bin/ln -s "./banner.openwrt" "./banner"
+  fi
+  exit 0
+endef
+
+define Package/ffnw-banner/install
+  $(INSTALL_DIR) $(1)/etc/
+  $(INSTALL_DATA) files/etc/banner.ffnw $(1)/etc/
+endef
+
+$(eval $(call BuildPackage,ffnw-banner))

banner/files/etc/banner.ffnw

+ _______              __   ___               __
+|    ___|.----.-----.|__|.'  _|.--.--.-----.|  |--.
+|    ___||   _|  -__||  ||   _||  |  |     ||    <
+|___|    |__| |_____||__||__|  |_____|__|__||__|__|
+ _______                __ Freie Netze Fuer alle! __
+|    |  |.-----.----.--|  |.--.--.--.-----.-----.|  |_
+|       ||  _  |   _|  _  ||  |  |  |  -__|__ --||   _|
+|__|____||_____|__| |_____||________|_____|_____||____|
+
+#######################################################
+  _______                     ________        __
+ |       |.-----.-----.-----.|  |  |  |.----.|  |_
+ |   -   ||  _  |  -__|     ||  |  |  ||   _||   _|
+ |_______||   __|_____|__|__||________||__|  |____|
+          |__| W I R E L E S S   F R E E D O M
+ -----------------------------------------------------
+ BARRIER BREAKER (Barrier Breaker, r43483)
+ -----------------------------------------------------