package/pkg-kconfig: hide defconfig targets if defconfig not supported

package/pkg-kconfig: hide defconfig targets if defconfig not supported

When the savedefconfig target is not supported by a kconfig package,
(like is the case for busybox) it doesn't make sense to define
busybox-savedefconfig or busybox-update-defconfig. Calling these leads
to an error from busybox itself "No rule to make target
'savedefconfig'.", which may be confusing.

Only define the savedefconfig and update-defconfig target if
$(2)_KCONFIG_SUPPORTS_DEFCONFIG is YES.

Note that we also need to define it as phony in the condition, otherwise
'make busybox-update-defconfig' will just say "Nothing to be done" and
we really want the error "No rule to make target
'busybox-update-defconfig'".

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>