tools/firmware-utils: enhance mktplinkfw utility

tools/firmware-utils: enhance mktplinkfw utility

Attached patch enhances the mktplinkfw utility:
1. existing firmware files in TP-Link format can now be inspected, showing eg. the used offsets
2. optionally the kernel and rootfs parts can be extracted (saves calculations for dd)
3. when generating firmwares, kernel entry point, load address and rootfs offset can be overwritten (for experienced users)

Patch-by: Pieter "Fate" Hollants <pieter@hollants.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@22169 3c298f89-4303-0410-b956-a3cf2f4a3e73