Makefile: fix distclean removal of $(O)

The distclean target no longer removes the "output" directory for
in-tree builds, because $(O) is no longer just "output" in that
case. Change the test to be against "$(CURDIR)/output", to match
the O setting, and a similar test elsewhere in the same Makefile.

Signed-off-by: Danomi Manchego <danomimanchego123@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>