Implemented support for proxydhcp variables (disbaled by default)

[notepass]

I've implemented the feature requested in #952
I'm not entirely sure if the type of implementation matches what the project wants. By default netboot.xyz will behave the same as before. Only after a change to "proxy-dhcp-vars.ipxe" is made, the new logic will be enabled.
The overall logic is as follows:

• Check if a proxydhcp is set
  • If no
    • Set the "next-server" provided by the normal DHCP as tftp server
  • If yes
    • Request proxy-dhcp-vars.ipxe from the next-server defined by the normal DHCP to attempt automatic conflict resolution
    • Check the content of the variable "force_use_root_dhcp_settings"
      • If it is true: Use the next-server provided by the normal DHCP server as tftp server
      • If it is false: Use the next-server provided by the proxy DHCP server as tftp server
      • If it is not set or another value: Prompt the user to press F5 to accept the settings from the proxy server. If no key is pressed in 4 seconds, the server provided by the normal DHCP will be used

This logic could be simplified by removing the additional config file, but than there could be changes for the end user (e.g. the newly added prompt), or the proxydhcp could be used whenever it is detected. The code can be changed to do either of this, I'm completely open for feedback on this.

Edit: The changes currently only impact UEFI booting, as BIOS booting doesn't seem to have access to proxydhcp info

[antonym]

Requested a few changes but I think the general idea makes sense.

[github-actions]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[notepass]

Sorry for the late reply, I was tageled up in tape drive stuff (heh).
I've implemented the requested changes

[antonym]

Have finally got around to setting up an environment to test with proxy working, so merging this in as is to build some images with it, test, and tune. I think the menu vars need to be reworked a bit as proxydhcp isn't a var thats set that's set but overall looks good. This should at least get us some builds to test and iterate on.