Update header for TRX to include versioning and offsets

1. Added display of loader/kernel/rootfs offsets based on version of the TRX firmware header.
2. Removed big endian TRX signature since the code converts to little endian. (Unless there are big endian TRX headers out in the wild?)

Also, why do all integer types default to signed by default? It seems that having unsigned as the default would make more sense.

Refs:
http://wiki.openwrt.org/doc/techref/header
https://dev.openwrt.org/browser/trunk/tools/firmware-utils/src/trx.c