Makefile: fix ignored trace at target-finalize

Makefile: fix ignored trace at target-finalize

Make may throw an error (but ignored) trace when cleaning up the
rootfs.

The target-finalize rule intends to remove the folder
`$(TARGET_DIR)/usr/share' but this directory may still contain items
(such as the `udhcpc' helper script) and causes the rmdir to fail.

The stderr output is redirected to /dev/null but it returns and error
which is escaped by the leading `-'; but make reports an ignored-error.

See the log below:
$ make
(...)
rm -rf (...)/target/usr/share/gtk-doc
rmdir (...)/target/usr/share
rmdir: failed to remove '(...)/target/usr/share': Directory not empty
make[1]: [Makefile:650: target-finalize] Error 1 (ignored)
find /(...)/target -type f \( -perm /111 -o -name '*.so*' \) -not \( -name 'libpthread*.so*' -o -name 'ld-*.so*' -o -name '*.ko' \) -print0 | xargs -0 (...)/host/usr/bin/arm-buildroot-linux-uclibcgnueabihf-strip --remove-section=.comment --remove-section=.note 2>/dev/null || true

This patch apply the same rule at the instruction immediately after:
* redirecting stderr to /dev/null (already done) and
* executing true if the `rmdir' instruction fails.

Signed-off-by: Gaël PORTAY <gael.portay@savoirfairelinux.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>