Allow to manage bootloader params "at use time", without recompilation

[pfalcon]

https://github.com/pfalcon/yaota8266/commit/1550d04652da235ea00d93a5ebfcec8cdafe7873 already introduced support for storing params (still represented in the code as C vars) at a predetermined offset in the image (boot8266, 1st sector of FlashROM).

The ultimate idea is to integrate with esptool, so user could run a command like boot8266_config dump and get dump of all params from a connected esp8266 device, e.g.:

gpio_mask: 0xf035
gpio_wait_ms: 3000

and a similar command to set params.

[pfalcon]

I have (had) initial scripts for this in works.