base-files: apply new sysctl.conf at postinst

This is mainly for kmod-br-netfilter.  To turn off
bridge-netfilter-call-xxx immediately after installation

While at it

 - Define filelist="/usr/lib/opkg/info/${pkgname}.list"
 - Reuse "[ -z "$root" ]"
 - Grep with "-m1"

Fixes FS#2300

Reported-by: Marco Sartorius <tidbits@ormoorgmen.info>
Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>

package/base-files/Makefile

@@ -12,7 +12,7 @@ include $(INCLUDE_DIR)/version.mk
 include $(INCLUDE_DIR)/feeds.mk
 
 PKG_NAME:=base-files
-PKG_RELEASE:=198
+PKG_RELEASE:=199
 PKG_FLAGS:=nonshared
 
 PKG_FILE_DEPENDS:=$(PLATFORM_DIR)/ $(GENERIC_PLATFORM_DIR)/base-files/