kbuild: Fix removal of the debian/ directory

scripts/Makefile.clean treats absolute path specially, but
$(objtree)/debian is no longer an absolute path since 7e1c0477 (kbuild:
Use relative path for $(objtree). Work around this by checking if the
path starts with $(objtree)/.

Reported-and-tested-by: Sedat Dilek <sedat.dilek@gmail.com>
Fixes: 7e1c0477 (kbuild: Use relative path for $(objtree)
Signed-off-by: Michal Marek <mmarek@suse.cz>