wic: Add --exclude-path option to rootfs source plugin.

wic: Add --exclude-path option to rootfs source plugin.

It will omit the given path from the resulting partition, and if the
given path ends in a slash, it will only delete the content, and keep
the directory.

Since mkfs only accepts whole directories as input, we need to copy
the rootfs directory to the workdir so that we can selectively delete
files from it.

Since we want to use the copyhardlinktree() function, we need to put
the generic oe lib in the module search path.

(From OE-Core rev: 6602392db3d391d926dead49fcc54326015cfe35)

Signed-off-by: Kristian Amlie <kristian.amlie@mender.io>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>