dmidecode: use absolute paths in fixup task

The calls to sed use relative paths so they depend on the directory the task
starts in.  To clarify the code, use absolute paths.

(From OE-Core rev: b61c56ade298726f0b861c5189cb9d5fd6260a36)

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-devtools/dmidecode/dmidecode_2.12.bb

@@ -12,7 +12,7 @@ do_install() {
 }
 
 do_unpack_extra() {
-	sed -i -e '/^prefix/s:/usr/local:${exec_prefix}:' Makefile
+	sed -i -e '/^prefix/s:/usr/local:${exec_prefix}:' ${S}/Makefile
 }
 addtask unpack_extra after do_unpack before do_patch