Allow boot-deploy config be customized by distros, relocate to /usr/share

The configuration from boot-deploy comes from the deviceinfo package. Adding another file where any kind of configuration can be added increases the risks of making mistakes and start pushing any kind of configuration to it. Since the file was born for other distros to be able to configure boot-deploy[1][2], let's reflect that in the file name. In the process, move the file to /usr/share from /etc, since it is not a user-owned (host- specific) but a distro-owned (should not be modified) file.

Also, update the documentation corresponding to the file, since both the file and all its variables are boot-deploy requirements.

[1] !18 (comment 1177011687) [2] !18 (comment 1177011689)

Revival of !24 (closed), with the "vendor" changed to "OS/distro" as appropriate.