package/pkg-kconfig: only define -update-{def, }config if supported

package/pkg-kconfig: only define -update-{def, }config if supported

The foo-update-config and foo-update-defconfig targets only work if a
custom config file was defined. Remove those targets entirely if they
don't work to begin with.

This was originally handled with an error condition in the
kconfig-package-update-config macro. However, it makes more sense to
simply remove the target if it anyway can't be used.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
[yann.morin.1998@free.fr: move PHONY before corresponding rule]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>