[SRU][PATCH v2 0/1][L][M][U] introduce do_lib_rust to enable/disable linux-lib-rust package

[Andrea Righi]

BugLink: https://bugs.launchpad.net/bugs/2021605

[Impact]

Provide a variable do_lib_rust=true|false in the kernel build system to
selectively enable the Rust packaging (linux-lib-rust) with specific
kernels and specific architectures.

Right now Rust should be enabled only in lunar and mantic and only on
amd64.

[Test case]

Build the kernel producing all the debs.

[Fix]

Introduce a do_lib_rust=true variable that will be defined only in
debian.master/rules.d/amd64.mk for now.

[Regression potential]

Kernels may need to set this variables if they want to enable the Rust
support.

ChangeLog (v1 -> v2):
 - add variable do_libc_rust to printenv
 - rewrite commit message to better describe the do_lib_rust variable