kbuild, deb-pkg: support overriding userland architecture

Usefull if building for sparc64 userland, because the
sparc and sparc64 userlands use the same 64-bit kernel,
making it impossible to always select the correct userland
architecture for the resulting debian package.

Might also be usefull, if you want a i386 userland with a amd64 kernel.

Example usage:
make KBUILD_DEBARCH=i386 deb-pkg

LKML-reference: <[e-mail]>
Signed-off-by: Asbjoern Sloth Toennesen <[e-mail]>
Reviewed-by: WANG Cong <[e-mail]>
Acked-by: maximilian attems <[e-mail]>
Signed-off-by: Michal Marek <[e-mail]>

Documentation/kbuild/kbuild.txt

···
 The output directory can also be specificed using "O=...".
 Setting "O=..." takes precedence over KBUILD_OUTPUT.
 
+KBUILD_DEBARCH
+--------------------------------------------------
+For the deb-pkg target, allows overriding the normal heuristics deployed by
+deb-pkg. Normally deb-pkg attempts to guess the right architecture based on
+the UTS_MACHINE variable, and on some architectures also the kernel config.
+The value of KBUILD_DEBARCH is assumed (not checked) to be a valid Debian
+architecture.
+
 ARCH
 --------------------------------------------------
 Set ARCH to the architecture to be built.

scripts/package/builddeb

···
 		echo "Please add support for $UTS_MACHINE to ${0} ..." >&2
 		echo "" >&2
 	esac
+	if [ -n "$KBUILD_DEBARCH" ] ; then
+		debarch="$KBUILD_DEBARCH"
+	fi
 	if [ -n "$debarch" ] ; then
 		forcearch="-DArchitecture=$debarch"
 	fi