util/kconfig: Provide default for DEFCONFIG

Our documentation claims that the DEFCONFIG make variable, used for
targets such as savedefconfig, defaults to 'defconfig'.
With the update to kconfig 5.13 we lost this default, so bring it back.

Fixes: 53ea1d44f0 ("util/kconfig: Uprev to Linux 5.13's kconfig")
Resolves: https://ticket.coreboot.org/issues/317
Change-Id: Idb88b69ffa855fa97df8c821601308e717575550
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/56718
Reviewed-by: Paul Menzel <paulepanter@mailbox.org>
Reviewed-by: Martin Roth <martinroth@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

util/kconfig/Makefile.inc

@@ -6,6 +6,8 @@ KBUILD_KCONFIG ?= src/Kconfig
 export LC_ALL=C
 export LANG=C
 
+DEFCONFIG ?= defconfig
+
 # Include original Makefile, with just enough edits to work for us
 $(objk)/Makefile.real: $(dir $(lastword $(MAKEFILE_LIST)))Makefile
 	mkdir -p $(objk)