osx packaging: switch background image to background.tiff

Also do a bit of cleanup:
 - Make the background name a variable so it's easier to change
 - Add proper make dependencies

Makefile.am

@@ -16,6 +16,7 @@ BITCOIN_WIN_INSTALLER=$(PACKAGE)-$(PACKAGE_VERSION)-win$(WINDOWS_BITS)-setup$(EX
 
 OSX_APP=Bitcoin-Qt.app
 OSX_DMG=Bitcoin-Qt.dmg
+OSX_BACKGROUND_IMAGE=background.tiff
 OSX_DEPLOY_SCRIPT=$(top_srcdir)/contrib/macdeploy/macdeployqtplus
 OSX_FANCY_PLIST=$(top_srcdir)/contrib/macdeploy/fancy.plist
 OSX_INSTALLER_ICONS=$(top_srcdir)/src/qt/res/icons/bitcoin.icns
@@ -30,7 +31,7 @@ WINDOWS_PACKAGING = $(top_srcdir)/share/pixmaps/bitcoin.ico \
   $(top_srcdir)/doc/README_windows.txt
 
 OSX_PACKAGING = $(OSX_DEPLOY_SCRIPT) $(OSX_FANCY_PLIST) $(OSX_INSTALLER_ICONS) \
-  $(top_srcdir)/contrib/macdeploy/background.png \
+  $(top_srcdir)/contrib/macdeploy/$(OSX_BACKGROUND_IMAGE) \
   $(top_srcdir)/contrib/macdeploy/DS_Store \
   $(top_srcdir)/contrib/macdeploy/detached-sig-apply.sh \
   $(top_srcdir)/contrib/macdeploy/detached-sig-create.sh
@@ -95,7 +96,7 @@ $(OSX_DMG): $(OSX_APP_BUILT) $(OSX_PACKAGING)
 deploydir: $(OSX_DMG)
 else
 APP_DIST_DIR=$(top_builddir)/dist
-APP_DIST_EXTRAS=$(APP_DIST_DIR)/.background/background.png $(APP_DIST_DIR)/.DS_Store $(APP_DIST_DIR)/Applications
+APP_DIST_EXTRAS=$(APP_DIST_DIR)/.background/$(OSX_BACKGROUND_IMAGE) $(APP_DIST_DIR)/.DS_Store $(APP_DIST_DIR)/Applications
 
 $(APP_DIST_DIR)/Applications:
 	@rm -f $@
@@ -106,11 +107,11 @@ $(APP_DIST_EXTRAS): $(APP_DIST_DIR)/$(OSX_APP)/Contents/MacOS/Bitcoin-Qt
 $(OSX_DMG): $(APP_DIST_EXTRAS)
 	$(GENISOIMAGE) -no-cache-inodes -D -l -probe -V "Bitcoin-Qt" -no-pad -r -apple -o $@ dist
 
-$(APP_DIST_DIR)/.background/background.png:
+$(APP_DIST_DIR)/.background/$(OSX_BACKGROUND_IMAGE): contrib/macdeploy/$(OSX_BACKGROUND_IMAGE)
 	$(MKDIR_P) $(@D)
-	$(INSTALL) $(top_srcdir)/contrib/macdeploy/background.png $@
-$(APP_DIST_DIR)/.DS_Store:
-	$(INSTALL) $(top_srcdir)/contrib/macdeploy/DS_Store $@
+	$(INSTALL) $< $@
+$(APP_DIST_DIR)/.DS_Store: contrib/macdeploy/DS_Store
+	$(INSTALL) $< $@
 
 $(APP_DIST_DIR)/$(OSX_APP)/Contents/MacOS/Bitcoin-Qt: $(OSX_APP_BUILT) $(OSX_PACKAGING)
 	INSTALLNAMETOOL=$(INSTALLNAMETOOL)  OTOOL=$(OTOOL) STRIP=$(STRIP) $(OSX_DEPLOY_SCRIPT) $(OSX_APP) -translations-dir=$(QT_TRANSLATION_DIR) -add-qt-tr $(OSX_QT_TRANSLATIONS) -verbose 2