ffmpeg: install ffserver and ffprobe when using custom versions

Signed-off-by: Ian Leonard <antonlacon@gmail.com>

multimedia/ffmpeg/Makefile

@@ -10,7 +10,7 @@ include $(TOPDIR)/rules.mk
 
 PKG_NAME:=ffmpeg
 PKG_VERSION:=3.4.6
-PKG_RELEASE:=1
+PKG_RELEASE:=2
 
 PKG_SOURCE:=$(PKG_NAME)-$(PKG_VERSION).tar.xz
 PKG_SOURCE_URL:=https://ffmpeg.org/releases/
@@ -752,11 +752,15 @@ define Package/ffprobe/install
 	$(CP) $(PKG_INSTALL_DIR)/usr/bin/ffprobe $(1)/usr/bin/
 endef
 
+Package/ffprobe-custom/install = $(Package/ffprobe/install)
+
 define Package/ffserver/install
 	$(INSTALL_DIR) $(1)/usr/bin
 	$(CP) $(PKG_INSTALL_DIR)/usr/bin/ffserver $(1)/usr/bin/
 endef
 
+Package/ffserver-custom/install = $(Package/ffserver/install)
+
 define Package/libffmpeg-custom/install
 	$(INSTALL_DIR) $(1)/usr/lib
 	$(CP) $(PKG_INSTALL_DIR)/usr/lib/lib{avcodec,avdevice,avformat,avutil}.so.* $(1)/usr/lib/