cmake: fix kconfig targets

Commit 269f350487 introduced a new
mechanism for loading the edtlib.EDT representation of the devicetree
from kconfigfunctions.py.

While this works for running kconfig.py from kconfig.cmake using
execute_process(), it doesn't work when kconfigfunctions.py is loaded
by the menuconfig target or any other Kconfig targets that use
add_custom_target().

The reason why is that these targets' environments do not have the
location of the EDT pickle file available. This is unlike kconfig.py,
which executes using the same environment as cmake itself, where
edt.pickle's location is already available in the environment.

Fix that by adding EDT_PICKLE to the environment for menuconfig and
friends.

Reported-by: Simon Guinot <simon.guinot@seagate.com>
Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>

cmake/kconfig.cmake

@@ -94,6 +94,7 @@ foreach(kconfig_target
     EXTRA_DTC_FLAGS=${EXTRA_DTC_FLAGS}
     DTS_POST_CPP=${DTS_POST_CPP}
     DTS_ROOT_BINDINGS=${DTS_ROOT_BINDINGS}
+    EDT_PICKLE=${EDT_PICKLE}
     ${PYTHON_EXECUTABLE}
     ${EXTRA_KCONFIG_TARGET_COMMAND_FOR_${kconfig_target}}
     ${KCONFIG_ROOT}