pkg-cmake: pass <pkg>_CONF_ENV for the target configure step

The Buildroot manual documents that <pkg>_CONF_ENV is passed in the
environment when calling cmake during the configure step. However, the
actual implementation in pkg-cmake passes HOST_<pkg>_CONF_ENV when
configuring the host variant of a cmake package, but does not pass
<pkg>_CONF_ENV when configuring the target variant of a cmake package.

This commit fixes that by passing <pkg>_CONF_ENV in the environment as
expected. It should not cause any behavior change, because this
feature is in fact not used by any package in upstream Buildroot:

$ grep CONF_ENV $(git grep -l cmake-package package/)
package/pkg-cmake.mk:$(2)_CONF_ENV			?=
package/pkg-cmake.mk:	$$($$(PKG)_CONF_ENV) $$(BR2_CMAKE) $$($$(PKG)_SRCDIR) \
package/pkg-cmake.mk:	$$($$(PKG)_CONF_ENV) $$(BR2_CMAKE) $$($$(PKG)_SRCDIR) \

This issue was reported by Olivier <ovalentin@awox.com> as bug #9616.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>