recipes: Switch away from S = WORKDIR

Where recipes use S = ${WORKDIR}, change them to set UNPACKDIR to
a subdir of WORKDIR and make S point at this instead.

I've chosen not to force S into any standard UNPACKDIR we may pick in
future just so the S = UNPACKDIR case is clearly visible by the
directory naming under WORKDIR as that should aid usability.

(From OE-Core rev: d9328e3b0b062f0621de3d114584b44f384a1c02)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-devtools/opkg/opkg-arch-config_1.0.bb

@@ -3,7 +3,8 @@ HOMEPAGE = "https://git.yoctoproject.org/opkg/"
 LICENSE = "MIT"
 PACKAGE_ARCH = "${MACHINE_ARCH}"
 
-S = "${WORKDIR}"
+S = "${WORKDIR}/sources"
+UNPACKDIR = "${S}"
 
 do_compile() {
 	mkdir -p ${S}/${sysconfdir}/opkg/