build: Unset CDPATH to avoid problems

In some places the output of commands, which include "cd" are used.
In case of CDPATH the new path is printed, which might not be expected.
Disable the variable to avoid these problem.

When CDPATH was set by the user to some value like "export CDPATH=."
the git checkout done by the build system did not work anymore, the
git cloning aborted with such an error message for example:
....
Packing checkout...
tar: /disk/fs1/tmp2/mehrtens/pon-ugw/ugw-haps/openwrt/tmp/dl/ppa-drv-1.0\n@1534240258: Cannot stat: No such file or directory
tar: Date sample file not found
Try 'tar --help' or 'tar --usage' for more information.
.....

To avoid this, this patch makes the build system unset CDPATH inside
the build system, so the build system will still work even when the
user set this variable in his local environment.

Signed-off-by: Thomas Langer <thomas.langer@intel.com>
Signed-off-by: Hauke Mehrtens <hauke.mehrtens@intel.com>
Acked-by: Hans Dedecker <dedeckeh@gmail.com>
(backported from 41faf52b0f)

Makefile

@@ -27,6 +27,8 @@ ifneq ($(OPENWRT_BUILD),1)
   export OPENWRT_BUILD
   GREP_OPTIONS=
   export GREP_OPTIONS
+  CDPATH=
+  export CDPATH
   include $(TOPDIR)/include/debug.mk
   include $(TOPDIR)/include/depends.mk
   include $(TOPDIR)/include/toplevel.mk