Makefile: Use CURDIR
Makefile: Use CURDIR

The current makefile variable PWD is not defined in the makefile,
and leads to build errors.  Use CURDIR instead.

Fixes errors like these:

  docker: Error response from daemon: Invalid volume spec ":/go/src/github.com/coreos/flannel:ro"

Signed-off-by: Geoff Levand <c3918f39022407dee6a0056ceca9b1aa80e8e4ce@infradead.org>

coreos/flannel,coreos/flannel,coreos/flannel