Rescue script (load firmware while booting with a btn pressed)

[spiccinini]

Provide a way to load a new firmware using a rescue script that is trigered when booting while pressing the button. Ideally it is an http server like in https://github.com/pepe2k/u-boot_mod but a simpler tftp command could be used.
Another idea is to enable netconsole when the btn is pressed.

[CodeFetch]

A router manufacturer is going to sell some routers without firmware (only bootloader). Thus I'm working on a bootloader that is easier to use including a DHCP server for the web interface, a mode where it acquires a DHCP lease and tries to load an image with a board specific filename from the gateway IP address and one where it loads an image using TFTP using a static IP address compatible to the stock U-Boots that support TFTP loading (e.g. TP-Link WR841N v9). The mode depends on how long you press the reset button. At the end the netconsole is being activated. I don't like the ordinary bootp flashing approaches for routers, that why I do things a bit different. Most of it is already working in my local tree. I'm going to inform you when I've pushed it.

The DHCPD is based on this implementation using the uIP stack like pepe2k's httpd.

[spiccinini]

@CodeFetch this is excellent!!! Please let me know if you need a hand! We can also enter this mode directly if no firmware can't be booted (because invalid checksum or there is no firmware like from your manufacturer)

[CodeFetch]

Exactly, when the firmware can't be loaded, the web interface is started... That's the plan.