[for-4.19][PATCH] build: Allow setting KBUILD_DEFCONFIG in the environment

[Michal Orzel <michal.orzel@amd.com>]

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 had no effect.

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

Signed-off-by: Michal Orzel <michal.orzel@amd.com>
---
In case of reluctance to this approach, we can always do:
 make -C xen defconfig KBUILD_DEFCONFIG=${KBUILD_DEFCONFIG}
in automation/scripts/build.

Also, Linux defers setting the variable to arch specific Makefile, so it is not
sth mandated by the top level Makefile or documentation.
---
 xen/Makefile | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/xen/Makefile b/xen/Makefile
index fd0e63d29efb..40feefff4ab5 100644
--- a/xen/Makefile
+++ b/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/.
-- 
2.25.1