kbuild: Make the toolchain variables easily overwritable

kbuild: Make the toolchain variables easily overwritable

Allow make variables to be overridden for each flavour by a file in
the build tree, .kernelvariables.

We currently use this for ARCH, KERNELRELEASE, CC, and in some cases
also CROSS_COMPILE, KCFLAGS.

This file can only be read after we establish the build tree, and all
use of $(ARCH) needs to be moved after this.

[bwh: Updated for 5.3: include .kernelvariables from current directory
 rather than using undefined $(obj).]

Gbp-Pq: Topic debian
Gbp-Pq: Name kernelvariables.patch