buildsys: ensure that all sources are copied over to the build directory

buildsys: ensure that all sources are copied over to the build directory

The copy command:
 # cp -a ${ZFSSRC}/* ${ZFSDIR}/
did not copied over hidden files (those beginning with a dot) and
thus a upstream patches which changed also .gitignore did not worked
when copied over 1:1, instead of removing the changes to .gitignore
it's IMO better to copy the full sources ensuring the whole state of
the upstream project source repo is used.

With submodules we still do not copy over a potential big .git
folder, as we as submodule user have the submodules .git state in our
.git/modules folder where the submodule just references to.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>