fetch2: handle absolute paths in subdir

fetch2: handle absolute paths in subdir

Currently if you use the subdir parameter in a SRC_URI and pass an absolute path
then it gets appended to the unpack directory instead of being used directly.
This is inconvenient as it may be useful to use ${S} when you want to unpack a
file into the source tree.

Change this behaviour so that absolute paths are used directly instead of being
appended to the root directory.  To ensure that recipes cannot write files to an
arbitrary location enforce that the subdir starts with the unpack root.

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