build: Allow setting KBUILD_DEFCONFIG in the environment

At the moment, in order to use a different defconfig target than default,
one needs to specify KBUILD_DEFCONFIG=<target> on the command line.
Switch to weak assignment, so that it can be also obtained from
environment similar to other KCONFIG/KBUILD variables.

This change will activate the use of KBUILD_DEFCONFIG variable in CI
build jobs that so far would have had no effect if they didn't match the
default anyway.

Note, that we will deviate from Linux in this regard.

Signed-off-by: Michal Orzel <michal.orzel@amd.com>
[stefano: adjust commit message]
Signed-off-by: Stefano Stabellini <stefano.stabellini@amd.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

xen/Makefile

@@ -256,7 +256,8 @@ export YACC = $(if $(BISON),$(BISON),bison)
 export LEX = $(if $(FLEX),$(FLEX),flex)
 
 # Default file for 'make defconfig'.
-export KBUILD_DEFCONFIG := $(ARCH)_defconfig
+# May be overruled on the command line or set in the environment.
+export KBUILD_DEFCONFIG ?= $(ARCH)_defconfig
 
 # Copy CFLAGS generated by "Config.mk" so they can be reused later without
 # reparsing Config.mk by e.g. arch/x86/boot/.