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-core/systemd/systemd-serialgetty.bb

@@ -9,7 +9,8 @@ SERIAL_TERM ?= "linux"
 
 SRC_URI = "file://serial-getty@.service"
 
-S = "${WORKDIR}"
+S = "${WORKDIR}/sources"
+UNPACKDIR = "${S}"
 
 # As this package is tied to systemd, only build it when we're also building systemd.
 inherit features_check