pythonnative: set PYTHON_EXECUTABLE so that cmake can find it

| CMake Error at /home/a.mueller/tmp/oe-core-glibc/sysroots/x86_64-linux/usr/share/cmake-2.8/Modules/FindPackageHandleStandardArgs.cmake:108 (message):
|   Could NOT find PythonInterp (missing: PYTHON_EXECUTABLE)

[RP: Added comment that is used by cmake]
(From OE-Core rev: 66455b8a0758176996028bf23f04391c7baf8abd)

Signed-off-by: Andreas Müller <schnitzeltony@googlemail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/pythonnative.bbclass

@@ -2,5 +2,7 @@
 inherit python-dir
 
 PYTHON="${STAGING_BINDIR_NATIVE}/${PYTHON_PN}-native/${PYTHON_PN}"
+# PYTHON_EXECUTABLE is used by cmake
+PYTHON_EXECUTABLE="${PYTHON}"
 EXTRANATIVEPATH += "${PYTHON_PN}-native"
 DEPENDS += " ${PYTHON_PN}-native "