bcm53xx: image: make filesystem a suffix for TRX files

This makes TRX files more /compatible/ with firmware files (e.g. CHK).
This will allow us to put some TRX files in BIN_DIR if we need to.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>


git-svn-id: svn://svn.openwrt.org/openwrt/trunk@42646 3c298f89-4303-0410-b956-a3cf2f4a3e73