feat: adding separate settings for automatic saving of the selected endpoint / ext_power state

[Timoyoungster]

Hello,

it's my first pull request so I hope I did everything right :)

This adds two new config values for en-/disabling the automatic saving of the selected endpoint / ext_power state. (#1599)

Per default both are set to y to keep the default behaviour the same.

I tested it on my own keyboard and as far as I can tell it works, but I'd be grateful if also other people could test it for their boards.

[hyx0329]

Hi, it looks the automatic saving is disabled, but the loading of those values are not disabled(if they exist due to an old firmware and not erased). It can be a feature XD, something disables automatic saving but allows manual saving.

Quick notes for anyone interested:

• search read_cb(cb_arg, &preferred_transport for transport(USB/BT) loading code
• search read_cb(cb_arg, &active_profile for BT profile loading code

My current approach is just commenting out the related code(so they are disabled) and that leads to a hacky but satisfying result :)

[Timoyoungster]

My primary reason for creating this PR was that I wanted to save write cycles to the eeprom's, since I sometimes switch a lot between bluetooth and usb when I'm working on my PC and laptop simultaneously.

Due to this regular switching I didn't really consider the default values and assumed that it would be set somewhere in the building/flashing process.

But if it's desired I could either add a config value for the defaults and disable the loading code or - as you mentioned - add a behavior to manually save the defaults.