Ensure DESTDIR will not confuse the build

Having DESTDIR defined will confuse the build of certain packages, so
we ensure that it is undefined from the environment when Buildroot
starts.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

Makefile

@@ -168,6 +168,10 @@ export HOSTCC_NOCCACHE HOSTCXX_NOCCACHE
 # Make sure pkg-config doesn't look outside the buildroot tree
 unexport PKG_CONFIG_PATH
 
+# Having DESTDIR set in the environment confuses the installation
+# steps of some packages.
+unexport DESTDIR
+
 # bash prints the name of the directory on 'cd <dir>' if CDPATH is
 # set, so unset it here to not cause problems. Notice that the export
 # line doesn't affect the environment of $(shell ..) calls, so