package/pkg-kconfig: Support custom $(MAKE)

package/pkg-kconfig: Support custom $(MAKE)

The U-Boot package requires GNU Make v4.0 or later, and so all U-Boot
"make" commands must use "$(BR2_MAKE)" so they use the host-make
package. Currently pkg-kconfig is hardcoded to uses $(MAKE), so add a
way to support $(BR2_MAKE). The package infra for pkg-automake and
pkg-cmake have a similar problem, and they solved it by defining a
$(PKG)_MAKE variable, and allowing each package to override it.

Signed-off-by: Brandon Maier <brandon.maier@rockwellcollins.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>